UNIVERSITY    OF    CALIFORNIA 

COLLEGE   OF   AGRICULTURE 

AGRICULTURAL    EXPERIMENT   STATION 

BERKELEY,  CALIFORNIA 


THE  DEHYDRATION  OF  PRUNES 


A.  W.  CHRISTIE 


BULLETIN  404 

August,  1926 


UNIVERSITY  OF  CALIFORNIA  PRINTING  OFFICE 

BERKELEY,  CALIFORNIA 

1926 


CONTENTS 

Introduction 3 

Development  of  dehydration  3 

Statistics  4 

Eelation  of  dehydration  to  sun-drying 5 

Comparative  quality    , 5 

Comparative  yield    7 

Comparative  costs    11 

Summary  of  comparisons 17 

Principles  of  dehydration  17 

Heat  requirements  18 

Fuels  :...  19 

Heating  systems 19 

Air  flow  requirements  20 

Methods  of  securing  air  flow  22 

Humidity  considerations  25 

Calculation  of  dehydrater  requirements  27 

Selection  of  a  dehydrater  29 

Dehydrater  manufacturers  30 

Patent   situation 31 

Some  construction  principles  34 

Arrangement  of  equipment  36 

Operation  of  dehydraters  37 

Dipping    37 

Green  grading  38 

Traying    39 

Temperature  40 

Humidity  42 

Drying  time  - 44 

Storage    45 

Summary  of  operating  methods  47 

Selected  references  47 


THE    DEHYDRATION    OF    PRUNES1 

A.  W.  CHRISTIE2 


INTRODUCTION 

California  is  the  leading  state  in  the  production  of  prunes, 
supplying  the  greater  part  of  the  domestic  consumption  in  the  United 
States  as  well  as  a  considerable  foreign  exportation.  The  total  acreage 
of  prunes  in  1925,  exclusive  of  the  plantings  of  that  year,  was  198,572 
acres,  of  which  73.6  per  cent  was  in  bearing.  The  production  in  1925 
amounted  to  145,000  dry  tons  and,  as  the  more  recent  plantings  come 
into  bearing,  the  annual  production  will  increase  considerably. 

Unlike  most  fruits,  prunes  are  rarely  sold  or  canned  fresh  but 
are  marketed  almost  entirely  in  the  dried  form.  Therefore,  drying 
is  a  most  important  operation  and  the  profitable  production  of  dried 
prunes  is  dependent  on  successful  drying  of  the  crop. 

i 

DEVELOPMENT  OF  DEHYDRATION 

Sun-drying  has  been  the  standard  and,  until  recent  years,  prac- 
tically the  only  method  of  drying  prunes  in  California.  Artificial 
drying  in  evaporators  was  common  during  the  early  years  of  the  prune 
industry  but  was  not  so  successful  at  that  time  as  the  sun-drying  which 
displaced  it.  While  the  adoption  of  improved  cultural  practices 
resulted  in  steady  improvement  of  the  quality,  size  and  yield  of  prunes, 
very  little  attention  was  given  to  improved  methods  and  equipment  for 
drying.  The  few  artificially  heated  dryers  which  had  been  used  were 
built  by  growers  with  little  knowledge  of  the  fundamental  principles 
of  dehydration.  As  a  result,  these  older  dryers  (see  Fig.  1)  were 
comparatively  inefficient  and  expensive  to  operate  and  drying  in  them 
was  slow  and  uneven.  Consequently,  they  were  only  used  to  supple- 
ment sun-drying  when  weather  conditions  prevented  natural  drying. 
The  popular  impression,  which  amounted  to  a  conviction  with  most 
growers,  was  that  artificial  drying  could  not  compete  with  sun-drying, 
either  in  quality  of  product  or  economy  of  operation.  The  fact  that 
all  prunes  produced  in  the  Pacific  Northwest  were  always  artificially 


i  This  bulletin  supersedes  Bulletins  330  and  337  in  so  far  as  they  concern  the 
construction  and  operation  of  dehydraters  for  prunes. 

2  Assistant  Professor  of  Fruit  Products,  Associate  Chemist  in  the  Experiment 
Station. 


4  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

dried,  because  of  the  impossibility  of  sun-drying  in  that  region, 
was  seemingly  disregarded  as  having  no  application  to  California 
conditions. 

The  interest  in  dehydration  and  its  development  during  the  world 
war  contributed  materially  to  an  understanding  of  the  economic 
application  of  the  principles  of  heating  and  ventilating  engineering 
to  the  evaporation  of  moisture  from  prunes.  The  unusually  heavy 
rains  of  September,  1918,  which  spoiled  prunes  worth  millions  of 
dollars,  greatly  stimulated  interest  in  dehydration.  The  work  of  state 
and  federal  investigators  was  instrumental  in  pointing  out  the  proper 


Fig.  1. — An  old-time  inefficient  dryer  with  " windmill' *  fan  and  gravity  oil 
burner.     (Note  man  stirring  prunes  on  sun-drying  trays.) 

construction  and  operation  of  dehydraters  and  in  showing  their 
advantages  over  the  natural  drying  of  prunes.  Growers  began  to 
appreciate  these  advantages  and  manufacturers  began  designing  and 
constructing  dehydraters  to  fill  the  growing  demand  for  their  use 
with  prunes  as  well  as  with  other  products. 


STATISTICS 

There  has  been  a  steady  growth  in  the  dehydration  of  prunes  in 
California,  particularly  since  1919,  as  illustrated  by  the  figures  in 
Table  1,  showing  the  increasing  number  of  dehydraters  and  the 
annual  tonnages  of  prunes  dried  therein.  A  census  of  all  dehydraters 
was  conducted  by  the  writer  during  1921,  1922  and  1923.  The  figures 
for  1924  and  1925  are  based  on  information  furnished  by  dehydrater 
manufacturers,  prune  packers  and  others,  and  are  believed  to  be 
approximately  correct. 


Bull.  404 


THE    DEHYDRATION    OF    PRUNES 
Table  1.— Growth  of  Prune  Dehydration 


Number  of 

dehydraters 

used  on  prunes 

Dry  tons 

Proportion 

Year 

Dehydrated 

Total  driedf 

dehydrated 

1921 

48 
126 
178 
244* 
310* 

2,946 
13,356 
16,810 
23,780* 
30,750* 

100,000 
132,000 
131,000 
139,000 
145,000 

2.9% 
10.1% 

1922 

1923 

12.8% 
17.1% 

21.2% 

1924 

1925 

*  Estimated. 


t  Compiled  by  Dried  Fruit  Association  of  California. 


Many  of  the  most  experienced  and  successful  prune  growers  have 
installed  and  are  continuing  to  use  dehydraters  to  the  exclusion  of 
sun-drying.  Their  success  with  this  modern  method  of  drying  is 
influencing  other  growers  to  make  use  of  the  advantages  of  dehydration. 


RELATION  OF  DEHYDRATION  TO  SUN-DRYING 

Success  in  drying  prunes  is  measured  by  three  main  results : 

1.  Production  of  the  finest  quality  of  dried  product  permitted  by 
the  nature  of  the  fruit  harvested. 

2.  Production  of   the   largest  size   and   greatest  weight   of  dried 
prunes  in  relation  to  the  composition  of  the  fresh  prunes. 

3.  Lowest  cost  of  drying  consistent  with  fine  quality   and  high 
yield. 

Therefore,  it  becomes  of  interest  to  consider  the  relative  merits  of 
sun-clrying  and  dehydration  with  respect  to  these  three  vital  results. 


COMPARATIVE  QUALITY 

The  satisfactory  drying  of  prunes  requires  not  only  the  reduction 
of  their  moisture  content  to  an  amount  which  prevents  spoiling  by 
molding  or  fermentation  but  also  certain  modifications  in  color  and 
flavor  which  have  become  trade  standards.  The  skin  should  be  black, 
the  flesh  a  light  amber  color  and  have  a  sweet  prune  flavor  free  from 
sourness  or  caramelization.  If  every  prune  season  consisted  through- 
out of  hot  dry  weather  conducive  to  rapid  drying,  there  would  not 
be  necessarily  any  significant  difference  between  naturally  and  artifi- 
cially dried  prunes.  However,  it  is  not  uncommon  for  part  of  the 
drying  season  to  consist  of  cold,  damp  weather  accompanied  by  fog 
or  showers.     In  some  years,  the  duration  of  such  unfavorable  drying 


b  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

weather  has  been  sufficient  to  cause  the  total  loss  of  a  considerable  part 
of  the  crop  through  molding  or  fermentation.  In  other  seasons  of  less 
unfavorable  weather,  actual  spoilage  losses  have  been  slight  but  a 
material  percentage  of  the  prunes  have  suffered  injury  to  the  color 
and  flavor  of  the  flesh  because  of  a  partial  fermentation  during  the 
time  drying  was  temporarily  arrested.  Such  injury  does  not  prevent 
the  sale  of  the  prunes,  but  definitely  lowers  their  quality  and  conse- 
quently their  market  value.  Furthermore,  the  percentage  of  prunes 
which  do  not  dry  promptly  or  properly  is  proportional  to  the  unfavor- 
able condition  of  the  weather  and,  as  a  result,  considerable  quantities 
of  variously  termed  "bloaters,"  "frogs,"  "chocolates,"  "slabs,"  etc., 
must  often  be  culled  out  to  maintain  the  quality  of  the  remainder 
of  the  crop.  This  not  only  causes  a  loss  in  yield  but  necessitates 
expensive  hand  sorting. 

Proper  dehydration  not  only  gives  rapid  and  continuous  evapo- 
ration of  the  excess  water  but  absolutely  prevents  deterioration  in 
quality  through  mold  or  fermentation.  Furthermore,  since  dehydration 
is  conducted  in  a  closed  building  by  currents  of  warm  air  free  from 
wind  blown  dust,  it  prevents  contamination  of  the  fruit  by  dirt  or 
insects  as  in  sun-drying.  In  brief,  dehydration  returns  a  clean, 
thoroughly  dried  product  fully  retaining  the  quality  of  the  original 
fruit.  Dehydration  is  in  line  with  the  modern  demand  for  sanitary 
production  of  foods. 

Not  all  dehydrated  prunes  have  been  of  the  best  quality.  In  some 
cases  inferiority  has  been  due  to  the  poor  quality  of  the  prunes  before 
dehydration.  Since  dehydration  is  merely  a  dependable,  controllable 
method  for  evaporating  the  excess  water  from  fruits  without  injury 
to  their  quality,  it  cannot  be  expected  to  improve  on  the  original 
qualities  of  the  fruit.  Rain  damaged  prunes,  salvaged  by  dehydration, 
should  never  be  classed  or  judged  as  dehydrated  prunes.  In  other 
cases,  inefficient  construction  or  operation  of  dehydraters  has  resulted 
in  injury  to  the  quality  of  the  prunes  during  dehydration  or  in 
deterioration  after  dehydration  because  of  improper  or  insufficient 
drying. 

However,  the  great  bulk  of  prunes  dehydrated  in  recent  years 
has  been  accepted  and  sold  as  being  of  the  best  market  quality.  An 
example  of  the  quality  of  dehydrated  prunes  is  seen  in  the  fact  that 
certain  orchardists  in  the  Sacramento  Valley,  who  previously  had 
never  obtained  the  highest  grade  for  their  dried  product,  have,  since 
the  installation  of  dehydraters,  consistently  obtained  the  highest  grade 
and  price  for  prunes  from  the  same  orchards  after  dehydration.  Cer- 
tain packers  freely  state  their  preference  for  the  dehydrated  product. 


BULL.  404  J  THE   DEHYDRATION    OF   PRUNES  7 

Other  packers,  because  of  unfavorable  experiences  with  occasional  lots 
of  improperly  dehydrated  prunes,  have  unjustly  condemned  dehydra- 
tion. Since  the  average  quality  of  dehydrated  prunes  is  unquestion- 
ably above  that  of  sun-dried  prunes,  it  would  be  more  logical  to 
condemn  sun-drying.  However,  since  the  quality  of  dehydrated 
prunes  has  become  more  generally  appreciated,  such  unwarranted 
prejudices  are  rapidly  disappearing  and  the  present  consensus  of 
opinion  is  that  the  quality  of  properly  dehydrated  prunes  is  at  all 
times  equal  to  and  often  superior  to  that  of  the  sun-dried. 


COMPARATIVE  YIELD 

It  has  long  been  known  that  if,  as  a  result  of  unfavorable  weather 
conditions,  prunes  undergo  a  partial  fermentation  during  sun-drying, 
the  yield  of  dried  product  is  materially  reduced.  This  is  explained  by 
the  fact  that  when  the  sugar  in  the  prunes  is  fermented  by  ever 
present  yeasts,  it  changes  into  alcohol  and  carbon  dioxide  gas.  Since 
both  these  compounds  are  volatile,  they  evaporate  into  the  surrounding 
air  and  thereby  cause  a  loss  in  weight  of  solid  matter  proportional 
to  the  extent  of  the  fermentation.  When  the  prunes  become  sufficiently 
dried,  the  action  of  micro-organisms  is  arrested  and  further  loss  in 
weight  on  this  account  is  prevented.  The  temperatures  normally  used 
in  dehydraters  are  above  the  temperatures  at  which  fermentation 
organisms  act  and  consequently  such'  losses  are  prevented  by 
dehydration. 

Therefore,  it  was  thought  that  when  fruit  underwent  rapid  and 
continuous  sun-drying  without  visible  signs  of  fermentation  that  no 
losses  other  than  water  evaporation  occurred  and  consequently  the 
maximum  possible  weight  of  dried  product  was  obtained.  Kecent 
investigations  show  that  even  under  the  most  favorable  sun-drying 
conditions  prunes  suffer  a  loss  in  sugar.  This  is  explained  by  the 
fact  that  the  living  tissues  of  prunes  contain  enzymes  which  cause 
respiration,  or  the  change  of  some  of  the  sugar  to  carbon  dioxide  gas 
and  water,  compounds  that  evaporate  from  the  prunes. 

As  long  as  the  fruit  remains  on  the  tree,  respiration  losses  are 
replaced  by  the  photosynthetic  action  of  the  leaves  which  manufacture 
sugar  for  translocation  to  the  fruit.  When  the  fruit  is  removed  from 
the  tree,  it  no  longer  absorbs  sugar  and,  since  the  tissues  remain  alive 
and  active  for  a  considerable  time  thereafter,  a  small  but  definite  loss 
of  sugar  occurs  during  the  long  sun-drying  and  does  not  entirely 
cease  until  the  prunes  are  nearly  dry.  While  the  brief  heating  incident 
to  lye-dipping  tends  to  reduce  subsequent  respiration  losses,  the  rapid 


8  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

drying  at  the  relatively  high  temperature  of  a  dehydrater  soon  kills 
living  tissues,  which  stops  respiration  and  consequently  prevents  loss 
in  weight  therefrom.  The  relatively  short  time  required  for  dehy- 
dration is  probably  of  greater  importance  in  minimizing  respiration 
losses  than  the  elevated  temperature. 

Comparative  Tests. — In  order  to  obtain  exact  data  on  the  relative 
yields  of  sun-dried  and  dehydrated  prunes,  comparative  field  tests 
were  made  in  several  prune  growing  districts, 

In  each  test  a  lot  of  prunes  varying  from  200  to  2000  pounds, 
freshly  harvested  from  a  restricted  area  of  orchard,  was  selected  in 
such  a  way  as  to  obtain  prunes  as  nearly  uniform  in  size  and  condition 
as  possible.  The  prunes  were  lye  dipped  in  the  customary  way  and, 
as  the  dipped  prunes  were  discharged  from  the  dipper  or  grader, 
they  were  received  alternately  on  sun-drying  and  dehydrater  trays, 
and  the  gross,  tare  and  net  weight  of  each  tray  recorded.  A  repre- 
sentative sample  of  the  fresh  prunes  was  sealed  in  a  jar  with  a  small 
amount  of  preservative  and  placed  in  freezing  storage  for  subsequent 
analysis. 

Sun-drying  was  conducted  by  exposing  the  prunes  to  direct  sun- 
shine until  about  three-fourths  dry  and  then  the  drying  completed 
in  the  stacked  trays.  Warm,  dry,  clear  weather  was  the  rule  in  every 
test  made  except  that  on  Robe  de  Sergeant  prunes  at  Visalia,  in 
which  case  rain  occurred  during  the  latter  part  of  the  drying  period 
and  the  prunes  suffered  considerable  fermentation  before  reaching 
dryness.  In  fact,  with  this  exception,  all  lots  were  dried  under 
optimum  sun-drying  conditions  for  the  locality  and  time  of  year. 

Dehydration  was  conducted  only  in  modern  air-blast  dehydraters 
operated  at  a  maximum  temperature  of  160°  to  165°  F.  The  time 
of  drying  varied  from  22  to  37  hours,  being  affected  primarily  by  the 
size  of  the  prunes.  As  soon  as  drying  was  complete,  the  net  weight 
of  each  lot  was  determined  and  a  representative  sample  placed  in  a 
sealed  jar  for  subsequent  analysis, 

Table  2  gives  the  yields  of  dried  prunes  per  100  pounds  of  fresh 
prunes  and  the  number  of  dried  prunes  per  pound,  first,  as  actually 
obtained  and,  second,  when  calculated  on  a  uniform  moisture  basis 
of  20  per  cent.  All  tests  gave  a  greater  weight  of  dehydrated  prunes. 
However,  in  every  case  the  increase  was  partly  due  to  a  higher  moisture 
content  left  in  the  dehydrated  prunes  and  when  this  variable  is 
eliminated,  the  increase  in  yield  is  less.  In  one  case  (Healdsburg) 
the  yield  is  actually  reversed  in  favor  of  sun-drying,  but  this  dis- 
crepancy is  probably  due  to  unevenness  in  the  prunes  or  an  unaccount- 
able error.     Dehydration  permits  exact  and  uniform  control  of  the 


Bull.  404] 


THE   DEHYDRATION    OF   PRUNES 


moisture  content  of  the  prunes  when  removed  from  the  dehydrater. 
As  a  result,  a  greater  weight  of  prunes  is  often  obtained  by  dehy- 
dration through  prevention  of  over-drying  which  frequently  occurs  in 
sun-drying.     Naturally,    wherever    the    dehydrated    prunes    gave    a 


Table  2.- 

—Comparative  Yields  of  Sun-Dried  and  Dehydrated  Prunes 

Date 

District 

Variety 

Test 
No. 

How  dried 

Per  cent 

water  after 

drying 

Sept.     8,  1922 
Sept.     8,  1922 
Sept.     8,  1922 

Cupertino 

Cupertino 

Cupertino 

French 

French 

French 

French 

Robe 

Robe 

Sugar 

1 
1 

2 
2 
3 
3 
4 
4 
5 
5 
6 
6 
7 
7 

Dehydrated 

Sun 

Dehydrated 

Sun 

26.3 
21.4 
22.9 

Sept.     8,  1922 
Sept.  27,  1923 

Cupertino 

Visalia 

20.1 

Dehydrated 

Sun 

Dehydrated 

Sun 

17.8 

Sept.  27,  1923 

Visalia 

17.3 

Sept.  15,  1923 

Cupertino 

19.9 

Sept.  15,  1923 

Cupertino 

Cupertino 

Sugar 

French 

French 

French 

French 

French 

French 

18.3 

Sept.  30,  1923 

Dehydrated 

Sun 

23.8 

Sept.  30,  1923 

Cupertino 

20.0 

Sept.  28,  1925 
Sept.  28,  1925 
Sept.     3,  1925 

Healdsburg 

Healdsburg 

Live  Oak 

Dehydrated 

Sun 

Dehydrated 

Sun 

28.9 
23.7 
15.9 

Sept.     3,  1925 

Live  Oak 

11.9 

Test 


1 — Dehydrated 

1— Sun 

2 — Dehydrated 

2— Sun 

3 — Dehydrated 

3— Sun 

4 — Dehydrated 

4— Sun 

5 — Dehydrated 

5— Sun 

6 — Dehydrated 

6— Sun 

7 — Dehydrated 

7— Sun 

Average  for  dehydra 
tion 

Average  for  sun  dry 
ing 


Yield  as  binned 


Drying 
ratio 


2.04 
2.36 
1.80 
1.89 
2.67 
39 
49 
69 
94 
12 
23 
31 
65 


3 

2. 

2. 

1 

2. 

2. 

2. 

2. 

2.86 


2.26 
2.52 


Pounds 

dry  per 

100  pounds 

green 


49.0 
42.4 
55.5 
52.9 
37.5 
29.5 
40.2 
37.2 
51*4 
47.2 
44.8 
43.3 
37.7 
35.0 


45.2 
41.1 


Count 

per 
pound 


40 
43 
43 
48 
51 
64 
32 
34 
39 
42 
89 
87 
60 
65 


51 
5.5 


Yield  on  20%  water  basis 


Drying 
ratio 


2.20 
2.40 
1.87 
1.89 
2.60 
3.28 
2.49 
2.63 
2.04 
2.12 
2.51 
2.42 
2.53 
2.60 


2.32 

2.48 


Pounds 

dry  per 

100  pounds 

green 


45.1 
41.6 
53.5 
52.9 
38.5 
30.5 
40.2 
38.0 
49.0 
47.2 
39.8 
41.3 
39.6 
38.5 


43.7 
41.4 


Count 

per 
pound 


43 
44 
45 
48 
50 
62 
32 
33 
41 
42 
100 
91 
57 
59 


53 
54 


10 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


greater  yield,  the  greater  weight  of  the  individual  prunes  is  reflected 
in  the  lower  count  per  pound.  # 

The  averages  of  all  tests  show  a  distinctly  greater  weight  and  size 
grade  of  prunes  as  a  result  of  dehydration  even  after  calculation  to  a 
uniform  moisture  basis. 


Table  3. — Changes  in  Weight  and  Composition  of  Prunes  After  Sun-Drying  and 

Dehydration 


Test 
No. 


Condition 


Total 
weight, 
pounds 


Pits, 
pounds 


Flesh, 
pounds 


Water, 
pounds 


Solids, 
pounds 


Sugar, 
pounds 


Solids 
not  sugar 
pounds 


Fresh 

Sun-dried 

Dehydrated.. 

Fresh 

Sun-dried 

Dehydrated.. 

Fresh 

Sun-dried 

Dehydrated.. 

Fresh 

Sun-dried 

Dehydrated.. 

Fresh 

Sun-dried 

Dehydrated.. 

Fresh 

Sun-dried 

Dehydrated.. 
Average : 

Fresh 

Sun-dried 

Dehydrated 


100.0 
42.4 
49.0 

100.0 
29.5 
37.5 

100.0 
37.2 
40.2 

100.0 
47.2 
51.4 

100.0 
43.3 
44.8 

100.0 
35.0 
37.7 

100.0 
39.1 
43.4 


5.0 
5.0 
5.0 
5.0 
5.0 
5.0 
5.0 
5.0 
5.0 
5.0 
5.0 


5.5 
5.0 
5.0 

5.1 
5.0 
5.1 


95.0 
37.4 
44.0 
95.0 
24.5 
32.5 
95.0 
32.2 
35.2 
95.0 
42.2 
46.4 
95.1 
38.4 
39.5 
94.5 
30.0 
32.7 


94. 
34. 

38. 


8.0 

11.6 

69.7 

4.2 

5.8 

68.8 

5.9 

7.0 

61.6 

8.5 

11.0 

65.0 

9.1 

11.4 

65.5 

3.6 

5.2 

66.1 
6.6 

8.7 


29. 

32. 

25. 

20. 

26. 

26. 

26. 

28. 

33. 

33. 

35. 

30. 

29. 

28. 

29.0 

26.4 

27.5 

28.8 
27.6 
29.7 


? 
18.9 
20.4 
15.4 
11.8 
15.8 
19.6 
18.7 
20.8 
24.1 
23.8 
24.9 
21.2 
20.4 
20.3 
17.5 
16.3 
17.5 

19.6 
18.3 
20.0 


? 

10.5 

12.0 

9.9 

8.5 

10.9 

6.6 

7.6 

7.4 

9.3 

9.9 

10.5 

8.9 

8.9 

7.8 

11.5 

10.1 

10.0 

9.0 
9.3 
9.8 


Table  3  shows  the  actual  pounds  of  pits,  moisture,  solids,  sugar, 
etc.,  in  the  fresh  prunes  and  the  relative  amounts  of  the  same  con- 
stituents remaining  after  sun-drying' and  dehydration.  All  figures  in 
Table  3  refer  to  100  pounds  of  fresh  prunes. 

An  examination  of  these  data  indicates  that : 

1.  Dehydration  results  in  a  greater  weight  of  dried  prunes  than 
sun-drying. 

2.  This  increase  is  in  part  due  to  a  greater  amount  of  water  retained 
but  there  is  also  a  greater  amount  of  sugar  retained  by  dehydration. 

3.  The  average  amount  of  sugar  remaining  after  dehydration  is 
approximately  the  same  as  in  the  fresh  prunes,  while  the  amount 
remaining  after  sun-drying  is  considerably  less. 


Bull.  404] 


THE   DEHYDRATION    OF   PRUNES 


11 


No  data  are  available  to  show  how  the  sugar  was  lost,  whether 
through  respiration  or  fermentation,  or  both.  Nevertheless,  the  data 
obtained  indicate  that  proper  dehydration  results  in  a  greater  total 
weight  and  slightly  larger  size  of  prunes  than  sun-drying.  This  is 
of  considerable  value  to  growers,  not  only  in  the  greater  total  weight 
of  dried  prunes  sold  but  in  the  higher  price  obtained  as  a  result  of 
larger  sizes.  Records  of  relative  yields  kept  by  a  number  of  growers 
support  this  observation  and  in  many  cases  the  increased  return  has 
been  sufficient  to  pay  the  cost  of  operating  the  dehydrater. 


COMPARATIVE   COSTS 

The  fine  quality  and  greater  yield  and  size  of  dehydrated  prunes 
would  probably  be  of  little  interest  to  growers  if  such  gains  were 
counterbalanced  by  a  greater  cost  for  dehydration.  Fortunately, 
however,  the  construction  and  operation  of  dehydraters  have  attained 
such  efficiency  that  the  advantages  of  dehydration  are  obtainable  at 
little  or  no  greater  total  cost  than  that  of  sun-drying  in  favorable 
weather,  while  in  unfavorable  weather  dehydration  is  often  less 
expensive. 

Table  4. — Comparative  Costs  of  Sun-Drying  and  Dehydrating  Prunes 
(Per  Fresh  Ton) 


Averages  for  dehydration 

Dehydrating1 

Sun-drying2 

Labor  (41.6c  per  hour): 

Dipping  and  traying — 1.86  hours 

$  .75 
.73 

.89 

$  .78 

Operating  dehydrater — 1.65  hours 

Scraping  trays — 2.19  hours 

3.11 

Total  labor — 5.70  hours 

$2.37 
.95 
.69 
.11 

$3.89 

Fuel  (5.08c  per  gallon) : 

Dipping  and  drying — 18.7  gallons 

Power  (2.14c  per  kilowatt  hour): 

Dipping  and  drying— 32.2  K.W.H 

.16 
.05 

Lye  (8.5c  per  pound): 

Dipping — 1.3  pounds 

.11 

Total  operating  cost 

$4.12 

$4.21 

1  Average  of  19  air-blast  dehydraters.  2  Average  of  11  dry-yards  (see  Bulletin  388). 

Operating  Costs. — In  order  to  present  reliable  figures  on  the  cost 
of  operating  prune  dehydraters,  exact  measurements  of  the  amount 
of  labor,  power,  fuel  and  lye  were  made  during  the  normal  operation 
of  nineteen  different  air-blast  dehydraters.     The  tests  included  all 


12  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

%SATI0NS  IN  COSTS  OF  DEHYDRATING 
PRUNES  PER,  FI^ESM  TON 

i  dollars. 4s. h **-\ i* 

]s 


Co  si  given  in 


63 


/.6  8 


1 — J0I 

III 

111 

.4  ell                                                    A90 

B=r.54 

■111 

iiiii 

16  6 

AS* 

I=.d 

IIP. 

/.99 

A4  7 


J.38 


ill 

111 

57 

a.av 

£1 

gl 


T^g 


I 


*.*e  U 


a.V8 


£.05 


.65 


^7^ 


Z.8f 


~1 


/A5* 


nil 

/Q7 


3.06 


I 

Fig.  2. — Variations  in  costs  of  dehydrating  prunes. 


Bull.  404]  THE  dehydration  of  prunes  13 

commercial  makes,  all  important  prune  districts  and  cover  the  past 
four  years.  The  plants  ranged  in  daily  capacity  from  4  to  40  fresh 
tons,  averaging  16.5  tons.  The  costs  cover  all  steps  in  the  process, 
beginning  with  dipping  and  ending  with  the  dried  prunes  in  storage 
6ins.  The  detailed  results  of  these  tests  are  presented  graphically  in 
Figure  2  and  summarized  in  Table  4.  Corresponding  figures  obtained 
from  eleven  dry -yards  (see  Bulletin  388)  are  also  included  in  Table  4. 
A  comparison  of  these  figures,  as  presented  graphically  in  Figure  3 
shows  the  average  costs  of  operation  for  dehydraters  and  dry-yards 
to  be  approximately  the  same.  The  cost  for  power  and  fuel  to 
operate  the  dehydrater  (average  of  $1.43  per  fresh  ton)  is  more  than 
counterbalanced  by  the  saving  in  labor  (average  of  $1.52  per  fresh 
ton)  aiforded  by  the  dehydrater. 

Average  cost  of  dehtoijating  -  U12  per  ton 


-B^ffiSg 


Labor  5.7  hours  -  *237 


AffiffAfiE  COST  OF  £tTN  DlffTNg     -  *4.21  TOR  TON 


Labor     10.3  hours     =   *3.&9 


Fig.  3. — Comparative  costs  of  sun-drying  and  dehydrating  prunes. 

As  previously  reported  in  Bulletin  337,  ''Some  Factors  of 
Dehydrater  Efficiency,"  published  in  November,  1921,  the  cost  of 
dehydrating  prunes  in  natural  draft  dehydraters  is  considerably  more 
than  in  air-blast  dehydraters.  Although  the  natural  draft  type  has 
no  charge  for  power,  the  expense  for  labor  and  fuel  is  higher  than 
in  air-blast  plants.  This  is  further  borne  out  by  more  recent  measure- 
ments made  on  three  commercially  built  natural  draft  dehydraters 
having  a  daily  capacity  of  3  to  4  fresh  tons  (see  Table  5). 

Table  5. — Cost  of  Operating  Natural  Draft  Prune  Dehydraters*  (Per  Fresh  Ton) 

Labor,  dipping,  drying,  scraping,  10.0  man  hours  at  39c $3.90 

Fuel,  20.4  gals,  at  16c 3.26 

Gasoline,  for  dipper  and  grader,  %  gal.  at  18c 06 

Lye,  1  pound  at  8c 08 

Total  operating  cost $7.30 

*  Average  of  three  dehydraters. 

Labor  Efficiency. — So  far  as  relative  costs  of  drying  are  concerned, 
the  greatest  advantages  of  the  dehydrater  lie  not  alone  in  the  fewer 
employees  required,  but  in  the  more  efficient  use  of  their  labor.  After 
the  prunes  are  dipped  and  trayed,  which  is  the  same  as  for  dehydration, 


14  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

prunes  for  sun-drying  are  moved  a  considerable  distance  into  the 
dry-yard  and  the  trays  then  spread,  one  at  a  time,  on  the  ground. 
Prunes  which,  because  of  large  size  or  unfavorable  weather,  dry 
unevenly,  must  be  stirred  to  promote  even  drying  and  minimize  the 
formation  of  "bloaters"  and  "slabs."  When  nearing  dryness,  the 
trays  are  customarily  stacked  on  the  ground.  When  judged  sufficiently 
dry,  the  trays  are  stacked  on  cars  and  moved  to  the  bin  house  for 
unloading.  It  is  also  frequently  necessary  to  pick  out  spoiled  or 
under-dried  prunes  before  the  rest  can  be  binned.  In  case  of  rain, 
the  labor  is  further  increased  because  of  additional  stacking,  spreading 
or  sorting. 

In  contrast  to  these  laborious  operations,  prunes  for  dehydration 
are  quickly  placed  in  the  dehydrater,  a  car  at  a  time,  and  when 
dehydrated  are  removed  and  the  trays  of  prunes  emptied  directly 
into  bins.  If  the  prunes  have  been  green  graded  and  uniformly  dried, 
no  sorting  or  re-drying  is  customarily  required.  One  experienced 
operator  on  the  day  shift,  and  one  on  the  night  shift,  can  keep  all  the 
operations  of  dipping,  dehydrating  and  binning  under  his  personal 
supervision,  which  is  difficult  in  large  sun-dry  yards.  Since  the 
dehydrater  dries  the  prunes  at  a  steady  rate,  the  workers  are  stimulated 
to  keep  pace  with  this  rate.  Such  stimulus  is  less  evident  in  sun-drying. 
Moreover,  workers  prefer  the  shaded,  restricted  area  of  the  dehydrater 
to  the  extended  area  and  hot  sun  of  the  dry  yard  and,  as  a  result, 
accomplish  more  work  and  do  it  more  cheerfully. 

For  these  reasons,  it  has  been  observed  that  not  only  are  fewer 
workers  needed  for  dehydration  but  that  each  accomplishes  more 
effective  work  than  in  sun-drying.  Since  the  growers'  greatest  problem 
during  the  harvest  season  is  adequacy  and  efficiency  of  labor,  a 
dehydrater  is  of  distinct  value  in  solving  this  problem. 

Fixed  Charges. — A  consideration  of  relative  costs  would  be  incom- 
plete without  including  comparative  fixed  charges  on  the  equipment 
required  in  the  two  methods  of  drying.  In  calculating  fixed  charges, 
it  is  necessary  to  include  interest  on  the  investment,  depreciation  and 
upkeep,  taxes  and  insurance  on  all  land,  buildings  and  equipment 
used  for  drying  or  handling  the  prunes  after  harvesting.  The  fixed 
charge  per  ton  of  prunes  dried  will  be  found  to  vary  considerably  in 
both  sun-drying  and  dehydration  with  the  nature  of  the  initial  invest- 
ment in  the  equipment  and  with  the  annual  tonnage  of  fruit  dried 
therewith. 

In  sun-drying,  the  much  greater  investment  in  land  and  trays 
often  balances  the  cost  of  a  dehydrater.  The  area  of  dry  yard  is 
normally  estimated  at  one  acre  for  each  20  acres  of  orchard,  while 


BULL.  404]  THE   DEHYDRATION    OF   PRUNES  15 

a  dehydrater  of  equal  capacity  occupies  only  about  5  per  cent  of  this 
area,  including  the  space  required  for  dipping  and  storing  the  fruit. 
While  some  growers  have  a  piece  of  land  of  little  or  no  agricultural 
value,  such  as  a  dry  creek  bottom,  which  makes  a  satisfactory  dry 
yard,  most  dry  yard  land  has  a  potential  value  equal  to  that  of  the 
surrounding  orchards.  While  there  is  usually  no  depreciation  on 
such  land  and  upkeep  costs  can  be  borne  by  some  annual  crop,  such 
as  hay,  grown  each  spring,  it  is  only  fair  to  include  interest  and  taxes 
on  the  dry  yard  acreage. 

Since  sun-drying  trays  are  rarely  used  more  than  twice  a  season, 
while  dehydrater  trays  are  usually  used  at  least  once  every  two  days, 
the  tray  surface  required  for  dehydration  is  only  10  to  15  per  cent 
of  that  required  for  sun-drying.  In  addition  to  interest  on  the  com- 
paratively heavy  investment  in  sun-drying  trays,  it  is  customary  to 
allow  at  least  10  per  cent  annually  for  depreciation  and  upkeep.  The 
nature  of  the  buildings  and  equipment  required  for  dipping  and 
binning  is  usually  independent  of  the  method  of  drying,  but  there  is 
considerably  more  trackage  used  in  the  dry  yard.  Insurance  is  often 
carried  on  buildings  and  trays.  Taking  all  these  factors  into  con- 
sideration, it  is  customary  to  figure  the  annual  fixed  charges  on 
sun-drying  equipment  at  from  15  to  20  per  cent  on  the  total  investment, 
which  investment  averages  $20  per  fresh  ton  of  prunes  dried  per 
annum.  According  to  figures  previously  presented  in  Bulletin  388, 
the  average  fixed  charge  for  sun-drying  prunes  is  $3.43  per  fresh  ton. 

In  dehydration,  interest  on  the  entire  investment  is  usually  placed 
at  6  or  7  per  cent.  Depreciation  and  upkeep  on  modern  types  of 
fireproof  dehydraters  is  rarely  figured  at  more  than  5  per  cent,  while 
on  trays  and  other  accessory  equipment  which  receive  hard  usage  it 
is  necessary  to  allow  10  per  cent  or  more.  A  charge  of  l1/^  Per  cent 
on  the  entire  investment  will  normally  be  adequate  to  cover  all  depre- 
ciation and  upkeep.  Taxes  are  variable  but  rarely  exceed  3  per  cent 
on  an  assessed  valuation  equal  to  50  per  cent  of  the  actual  value. 
Many  owners  of  fireproof  dehydraters  carry  no  insurance,  while  some 
carry  insurance  only  on  the  wooden  buildings,  trays,  etc.,  used  in 
connection  with  the  dehydrater. 

In  the  case  of  some  of  the  older  inefficient  dehydraters  built  several 
years  ago  which  have  since  been  either  dismantled  or  remodeled,  a 
heavy  charge  for  obsolescence  was  incurred.  However,  the  present 
leading  types  of  commercially  built  dehydraters  have  become  so 
standardized  and  have  proved  so  efficient  that  it  is  probably  not 
necessary  to  include  a  charge  for  obsolescence. 

Adding  these  charges  together,  it  is  found  that  the  total  fixed 
charge  on  a  substantially  built  fireproof  dehydrater,  together  with 


16 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


accessory  buildings  and  equipment,  will  average  16  to  17  per  cent 
per  annum  on  the  investment,  which  investment  averages  $22  per 
fresh  ton  of  prunes  dried  per  annum. 

The  detailed  figures  on  fixed  charges  in  6  modern  air-blast 
dehydraters  given  in  Table  6  show  an  average  fixed  charge  of  $3.65 
per  ton  for  dehydrating.     Comparing  this  with  the  corresponding 

Table  6. — Fixed  Charges  on  Prune   Dehydraters    (Per  Fresh  Ton) 


Plant 

Total 
investment 

Season 
tonnage 

Interest  at 
7% 

Deprecia- 
tion at 

71% 

Taxes 

Insurance 

Total 

S 

B 

$13,000 

12,850 

6,000 

7,000 

6,770 

19,300 

575 
524 
399 
521 
201 
733 

$1.58 
1.72 
1.05 

|li.94 
2.46 
1.84 

$1.69 
1.84 
1.12 
1.01 
2.64 
1.97 

$    .40 
.37 
.16 

.20 
.27 
.39 

$  .18 

$3.85 
3.93 

A 

2.33 

W 

2.15 

K 

5.37 

E 

.04 

4.24 

Average 

$10,820 

492 

$1.60 

$1.71 

$  .30 

$  .04 

$3.65 

Fig.  4. — The  first  Puccinelli  Dehydrater  built  (1921).    Note  young  prune  trees  in 

former  dry  yard. 

average  of  $3.43  for  sun-drying,  it  is  seen  that  fixed  charges  for 
dehydration  are  only  slightly  greater  than  for  sun-drying.  Adding 
the  comparative  operating  costs  given  in  Table  4  to  the  corresponding 
fixed  charges,  it  is  found  that  the  average  total  cost  of  dehydrating 
is  $7.76  per  fresh  ton  as  compared  with  $7.64x  for  sun-drying,  which 
difference  is  not  significant.  Using  the  even  amount  of  $8  a  fresh 
ton  and  an  average  drying  ratio  of  2.5  to  1,  it  is  evident  that  the  cost 
of  drying  prunes  is  approximately  one  cent  a  dry  pound.    It  may  be 


1  One  of  the  oldest  and  largest  non-profit  cooperative  dry  yards  in  Santa 
Clara  County  makes  a  charge  of  $8  a  ton  for  drying  and  storing  prunes. 


BULL.  404]  THE   DEHYDRATION   OF   PRUNES  17 

concluded  that  the  average  total  cost  of  dehydration  is  no  greater  than 
for  sun-drying  if  based  on  present  prices  for  complete  new  equipment 
in  each  case. 

Growers,  who  already  have  adequate  sun-drying  equipment  in  most 
cases  purchased  at  considerably  less  than  present  prices,  may  find 
the  investment  and  therefore  the  fixed  charges  on  a  new  dehydrater 
to  be  temporarily  somewhat  greater  than  on  their  dry  yard.  However, 
many  such  growers  have  installed  dehydraters,  sold  most  of  their 
trays  and  made  their  dry  yard  land  more  profitable  by  planting  it  to 
trees  (see  Fig.  4),  feeling  that  the  economic  advantages  of  dehydration 
are  more  than  sufficient  to  balance  a  slightly  greater  fixed  charge. 

Growers  who  must  provide  new  or  additional  drying  equipment 
for  young  orchards  coming  into  bearing  will,  with  few  exceptions, 
find  it  advantageous  to  employ  dehydration  rather  than  sun-drying. 

SUMMARY  OF   COMPARISONS 

Summarizing  the  foregoing  comparisons,  it  can  be  fairly  said  that 
dehydration  produces  prunes  of  equal  or  better  quality  than  the  sun- 
dried,  generally  results  in  a  greater  yield  and  size  of  prunes,  provides 
insurance  against  rain  damage  losses,  and  the  total  cost  of  operating 
an  efficient  dehydrater,  including  fixed  charges,  need  be  no  greater 
than  for  a  dry  yard  of  equal  capacity.  Consequently,  growers  who 
can  finance  the  installation  of  a  dehydrater  or  who  can  have  their 
prunes  dehydrated  at  a  reasonable  custom  charge  will  find  it  to  their 
financial  advantage  to  adopt  this  modern  method  of  drying  as  many 
growers  already  have  done. 


PRINCIPLES  OF  DEHYDRATION 

Dehydration  may  be  defined  as  the  evaporation  of  water  from 
substances  in  a  current  of  air,  the  temperature,  humidity  and  flow  of 
which  are  subject  to  control.  The  fundamental  laws  of  physical 
science  on  which  dehydration  is  based  have  long  been  known  and  used 
in  heating  and  ventilating  engineering.  The  practical  application  of 
these  principles  to  the  dehydration  of  fruits  has  already  been  presented 
in  detail  in  several  technical  publications  (see  list  of  references, 
page  47.  Since  very  few  growers  build  their  own  dehydraters,  it 
is  not  necessary  in  this  bulletin  to  give  all  details  governing  design 
and  construction.  However,  in  order  that  growers  may  have  the 
necessary  information  to  intelligently  select  and  operate  a  dehydrater, 
the  following  brief  elementary  presentation  of  the  principles  of 
dehydration  is  given. 


18  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

HEAT    REQUIREMENTS 

By  evaporation  of  water  is  meant  the  change  of  water  from  the 
liquid  to  the  vapor  state.  To  accomplish  this  change  requires  the 
expenditure  of  a  definite  amount  of  heat.  In  sun-drying,  this  heat  is 
derived  from  the  sun,  while  in  dehydration  it  is  produced  by  the 
combustion  of  fuel.  The  unit  used  to  measure  heat  in  dehydration 
is  the  British  Thermal  Unit  (hereafter  referred  to  as  B.T.U.)  which 
represents  the  amount  of  heat  absorbed  in  raising  the  temperature 
of  one  pound  of  water  one  degree  Fahrenheit. 

If  prunes  at  a  temperature  of  60°  F.  are  placed  in  a  dehydrater 
and  dried  at  an  average  temperature  of  150°  F.,  90  B.T.U.  will  be 
required  to  raise  the  temperature  of  each  pound  of  water  in  the  prunes 
from  60°  to  150°  F.  The  heat  required  to  transform  one  pound  of 
water  from  the  liquid  to  the  vapor  state,  at  150°  F.  is  1010  B.T.U. 
Consequently,  the  total  amount  of  heat  theoretically  required  to  heat 
the  water  in  the  prunes  to  the  average  temperature  of  the  dehydrater 
and  then  evaporate  that  water  is  90  plus  1010  or  1100  B.T.U.  per 
pound  of  water  evaporated.  The  actual  amount  of  heat  theoretically 
required  will  vary  somewhat  with  the  original  temperature  of  the 
prunes  and  the  average  temperature  at  which  the  dehydrater  is 
maintained.  However,  for  practical  purposes,  1100  B.T.U.  per  pound 
of  water  evaporated  may  be  taken  as  the  basic  requirement. 

Heat  must  also  be  provided  for  other  purposes.  /The  walls  and 
roof  of  the  dehydrater  are  constantly  radiating  heat  which  must  be 
replaced  in  order  to  maintain  the  dehydrater  at  the  desired  tempera- 
ture. The  solid  matter  of  the  fruit  and  the  trays  and  cars  which 
carry  the  fruit  enter  the  dehydrater  cold  and  emerge  at  the  maximum 
temperature  and  consequently  carry  away  the  heat  which  they  have 
absorbed.  Heat  is  lost  from  the  smoke  stack  in  order  to  provide  the 
draft  necessary  to  combustion  of  the  fuel  and  the  radiation  of  the  heat 
generated.  Heat  is  also  lost  with  the  warm  exhaust  air  which  removes 
the  water  vapor  and  often  through  cracks  in  the  building  or  around 
doors.  None  of  these  losses  can  be  entirely  eliminated  but  all  can 
be  minimized  by  proper  construction  and  operation.  The  overall 
fuel  efficiency  of  prune  dehydraters  has  been  found  to  vary  consider- 
ably with  the  type  of  construction  and  with  the  relative  temperatures 
of  the  inside  and  outside  air.  Numerous  tests  show  that  efficiently 
constructed  and  operated  dehydraters  generally  give  an  overall  fuel 
efficiency  of  40  per  cent  or  higher,  sometimes  as  high  as  50  per  cent. 
Taking  45  per  cent  efficiency  as  an  average  figure,  the  total  heat 
requirement  is  1100-^0.45,  or  2444  B.T.U.  per  pound  of  water 
evaporated. 


BULL.  404]  THE   DEHYDRATION   OF   PRUNES  19 

FUELS 

Of  the  common  sources  of  heat,  oil  is  the  most  convenient  and 
economical  in  California  for  dehydrating  prunes  and  is  therefore 
almost  universally  used.  Wood  and  coal  necessitate  additional  expense 
for  handling  and  unless  used  as  a  source  of  steam  heat  cannot  be 
easily  controlled.  Cheap  natural  gas,  while  an  excellent  fuel,  is  not 
available  in  most  prune  districts.  Electricity  is  a  convenient  and 
efficient  source  of  heat  but  even  at  a  rate  as  low  as  one  cent  per 
kilowatt  hour,  the  cost  of  electrical  heat  for  dehydration  is  prohibitive. 

HEATING    SYSTEMS 

There  are  three  types  of  heating  systems:  direct  heat,  direct 
radiation  and  indirect  radiation. 

Direct  heat  means  the  absorption  of  the  heat  from  the  burning 
fuel  by  the  air  used  to  dry  the  prunes,  without  the  intervention  of 
furnace  walls  or  flues.  The  hot  gases  from  the  combustion  of  oil  or 
gas  are  drawn  into  and  mixed  with  the  main  air  stream  in  such 
proportion  as  to  give  the  resultant  mixture  the  desired  temperature. 
The  advantages  of  this  system  are : 

1.  Reduction  in  fuel  consumption  through  elimination  of  stack 
losses. 

2.  Lower  cost  of  installation. 

3.  Reduced  depreciation  and  upkeep  charges  as  compared  with 
radiation  systems. 

Common  disadvantages  of  this  system  have  been : 

1.  The  use  of  higher  priced  partially  refined  oils  to  insure  complete 
combustion. 

2.  The  potential  danger  of  contaminating  the  fruit  with  unburned 
fuel  or  soot. 

While  there  have  been  a  few  cases  of  fruit  being  injured  by  this 
method,  many  thousands  of  tons  of  prunes  have  been  successfully 
dehydrated  by  direct  heat.  Observations  show  no  consistent  differ- 
ence in  quality  of  fruit  and  fuel  charges  between  the  direct  heat 
and  direct  radiation  systems  at  present  in  use,  but  indications  point 
to  lower  upkeep  costs  for  the  direct  heat  system. 

Direct  radiation  means  the  radiation  of  heat  through  the  metal 
walls  of  furnaces  and  flues  directly  into  the  air  used  in  drying.  This 
is  the  system  in  most  common  use  for  fruit  dehydraters.  If  properly 
constructed,  this  system  prevents  possible  contamination  of  fruit  b}^ 
unburned  fuel  and  gives  relatively  high  fuel  efficiency.  Its  sole  dis- 
advantage has  been  the  occasional  replacement  of  burnt  out  flues, 


20  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

especially  those  nearest  the  high  temperature  of  the  furnace.  By 
using  flues  of  such  length  and  radiating  surface  that  the  stack  tempera- 
ture is  as  low  as  is  consistent  with  adequate  draft,  furnace  efficiencies 
of  70  to  80  per  cent  are  possible  with  this  system. 

Indirect  radiation  means  that  the  heat  from  the  fuel  is  transferred 
to  the  drying  air  through  the  intermediate  agency  of  a  steam  boiler 
and  steam  heating  coils.  Possible  advantages  of  this  system  are  that 
any  kind  of  fuel  can  be  used  and  the  temperature  of  the  drying  air 
automatically  controlled  by  a  thermostatic  steam  valve.  The  dis- 
advantages of  the  steam  heating  system  are  its  relatively  greater  first 
cost  and  the  fact  that  it  can  not  at  best  give  an  air  heating  efficiency 
of  over  50  to  60  per  cent. 

Thermal  Efficiency. — The  over  all  thermal  efficiency  can  be  easily 
calculated  by  use  of  the  following  formula: 

Pounds  water  evaporated  X  1100  B.T.U.  w  _._         «,*»,««. 
Gallons  oil  consumed  X  142,000  B.T.U.  X  10°  =  %  fuel  efflcienCy- 

The  pounds  of  water  evaporated  during  a  given  time,  usually  24  hours, 
is  determined  from  the  difference  in  weight  between  the  fresh  fruit 
entering  and  the  dried  fruit  leaving  the  dehydrater.  The  number  of 
gallons  of  oil  consumed  during  the  same  time  can  be  calculated  from 
the  following  formulae : 

Vertical  Cylindrical  Tanks — 

3.1416  X  %  diam.  X  %  diam.  X  drop  in  level,  (all  in 
inches  -f-  231  =  gallons) . 
Rectangular  Tanks — 

Inside  width  X  inside  length  X  drop  in  level,   (all  in 
inches  -=-  231  =  gallons) . 

AIR   FLOW   REQUIREMENTS 

Air  performs  two  essential  functions  in  a  dehydrater.  First,  it 
conducts  the  heat  from  the  air  heating  system  to  the  fruit  which  is  to 
be  dried  and,  second,  it  absorbs  and  removes  the  water  vapor  which 
that  heat  has  evaporated  from  the  fruit.  It  is  obvious,  therefore,  that 
the  capacity  of  any  dehydrater  to  dry  prunes  depends  not  alone  on 
the  temperature  of  the  air  but  more  particularly  on  the  volume  of 
heated  air  which  is  brought  into  contact  with  the  prunes.  More 
dehydraters  have  failed  to  give  the  expected  capacity  or  efficiency 
because  of  inadequate  air  flow  than  from  all  other  causes  combined. 
For  accuracy  in  dehydrater  calculations  air  must  be  considered  as 
a  mixture  of  dry  air  and  water  vapor,  both  of  which  contain  heat. 


Bull.  404]  THE  dehydration  of  prunes  21 

When  this  air  mixture  comes  in  contact  with  moist  prunes,  a  drop  in 
the  temperature  of  the  air  takes  place,  indicating  that  part  of  the 
heat  in  the  air  mixture  has  been  used  in  changing  water  in  the  prunes 
from  the  liquid  to  the  vapor  state.  Consequently,  the  amount  of 
evaporation  which  takes  place  depends,  first,  on  the  drop  in  tempera- 
ture of  the  air,  and,  second,  on  the  volume  of  that  air  passing  over  the 
prunes  in  a  given  time. 

For  example,  let  it  be  assumed  that  air  enters  the  drying  chamber 
at  a  temperature  of  165°  F.,  and  a  relative  humidity  of  25  per  cent. 
By  reference  to  tables  of  composition,  one  cubic  foot  of  this  air  is 
found  to  contain  .0578  pounds  of  dry  air  and  .0036  pounds  of  water 
vapor  or  a  total  of  .0614  pounds  of  air  mixture  per  cubic  foot.  The 
amount  of  heat  which  this  air  can  give  up  in  dropping  one  degree  is 
determined  by  multiplying  the  pounds  of  dry  air  and  of  water  vapor 
by  their  respective  specific  heats1  and  adding  these  two  results  together. 
This  gives  .0155  B.T.U.  as  the  amount  of  heat  given  up  by  one  cubic 
foot  of  this  air  mixture  in  dropping  1°  F.  Now  if  the  total  number  of 
B.T.U.  required  per  minute  to  evaporate  the  water  from  a  given 
weight  of  prunes  in  a  given  number  of  hours,  together  with  the  heat 
lost  from  the  drying  chamber  by  radiation,  air  discharge,  etc.,  be 
known  or  estimated,  it  is  possible  to  calculate  the  cubic  feet  of  air 
required  by  multiplying  .0155  B.T.U.  by  the  number  of  degrees  the 
air  drops  in  temperature  in  passing  through  the  dehydrater  and 
dividing  this  result  into  the  number  of  B.T.U.  required  per  minute 
for  evaporation.  This  gives  the  cubic  feet  of  air  per  minute  which 
must  enter  the  drying  chamber.  For  example,  if  the  above  air  mixture 
in  passing  through  the  drying  chamber  has  a  temperature  drop  of  35°, 
each  cubic  foot  of  air  will  give  up  35  times  .0155  or  .5425  B.T.U. 
of  heat.  Since  each  pound  of  water  evaporated  theoretically  requires 
1100  B.T.U.  the  volume  of  air  which  must  pass  through  the  drying 
chamber  to  evaporate  one  pound  of  water  per  minute  would  be  1100 
divided  by  .5425  or  2028  cubic  feet  per  minute.  However,  a  part  of 
the  heat  given  up  by  this  air  will  not  be  available  for  evaporation 
because  it  will  be  lost  by  radiation  or  leaks  or  in  heating  trays,  cars, 
etc.  Consequently,  an  additional  amount  of  air  must  be  provided  to 
compensate  for  these  heat  losses.  Assuming  the  air  to  have  an  actual 
evaporating  efficiency  of  75  per  cent,  2704  cubic  feet  of  air  per  minute 
will  be  required  for  each  pound  of  water  to  be  evaporated  per  minute. 


i  Specific  Beat  is  the  ratio  between  the  heat  required  to  raise  (or,  conversely, 
given  off  by  cooling)  one  pound  of  any  substance  1°  F.,  and  that  required  to 
raise  one  pound  of  water  1°  F.,  the  specific  heat  of  water  being  considered  as  1. 
The  specific  heat  of  dry  air  and  of  water  vapor  are  taken  as  .24  and  .45 
respectively. 


22  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 


METHODS  OF  SECURING  AIR  FLOW 

Natural  draft  is  the  oldest  and  simplest  method  of  inducing  air 
flow.  It  depends  on  the  fact  that  when  air  is  heated  it  becomes  lighter 
because  of  expansion  and  tends  to  rise,  thereby  creating  an  upward 
current  of  air.  The  advantage  of  this  system  is  that  it  does  not  require 
the  use  of  power  driven  fans,  the  energy  required  for  air  movement 
being  furnished  by  the  burning  of  additional  fuel.  The  disadvan- 
tages are : 

1.  Inadequate  volume  and  velocity  of  air  for  all  but  small  units. 

2.  Lack  of  control  of  air  distribution  causing  uneven  drying. 

3.  Difficulty  of  securing  quick  and  exact  control  of  temperature 
and  humidity. 

The  natural  draft  system  reached  its  most  extensive  development 
in  the  ''Oregon  Tunnel"  dryers,  but  in  recent  years  many  of  these 
have  been  remodeled  to  the  recirculating  fan  system  in  order  to  obtain 
the  increased  capacity  and  economy  of  the  latter.  Natural  draft  dryers 
are  now  only  used  by  growers  with  small  tonnages.  Their  total  cost 
of  operation,  including  fixed  charges,  is  usually  much  greater  than 
that  of  fan  equipped  dehydraters. 

Air  blast  dehydraters  are  those  in  which  the  air  flow  is  produced 
by  power  driven  fans.  The  air  flow  is  mainly  in  a  horizontal  direction 
over  the  trays.  The  advantages  of  this  system  are  that  it  permits  exact 
control  of  the  temperature,  humidity,  volume  and  distribution  of  the 
air.  Although  requiring  a  considerable  investment  in  one  or  more 
fans  and  motors,  this  extra  cost  is  well  repaid  by  more  rapid  and 
uniform  drying  and  greater  economy. 

The  fans  used  in  dehydraters  are  of  two  main  types:  disc  and 
propeller  fans  which  have  8  to  14  blades  about  a  central  hub  blow  air 
in  a  direction  parallel  to  the  fan  shaft;  centrifugal  or  multivane 
fans  which  have  48  or  more  short  blades  on  a  wheel  which  revolves 
within  a  housing  and  blows  air  at  right  angles  to  the  fan  shaft.  The 
steel  plate  fan  is  similar  but  has  fewer  and  larger  blades. 

Dehydraters  of  small  capacity  or  those  with  a  series  of  fans  for 
supplying  air  to  several  sections  have  used  disc  or  propeller  fans 
satisfactorily.  In  large  dehydraters  where  a  large  volume  of  air  must 
be  circulated  through  a  long  drying  chamber,  the  multivane  fan  is 
the  most  efficient  because  of  its  ability  to  produce  an  adequate  flow  of 
air  against  relatively  high  frictional  resistance. 

Position  of  Fans. — This  is  determined  by  convenience  in  the  par- 
ticular design  of  dehydrater  adopted.  The  position  of  the  fan  is  also 
determined  by  the  preference  of  the  designer  for  either  blowing  the 


Bull.  404]  THE  dehydration  of  prunes  23 

air  through  the  drying  chamber  under  slight  pressure  or  drawing  the 
air  through  the  drying  chamber  under  slightly  reduced  pressure. 
There  are  two  usual  positions  in  which  the  fan  is  placed: 

1.  Drawing  direct  from  the  drying  chamber  and  returning  the 
recirculated  air  by  blowing  it  through  the  heating  chamber. 

2.  Blowing  directly  through  the  drying  chamber  and  returning  the 
recirculated  air  by  drawing  it  through  the  heating  chamber. 

The  first  system  tends  to  draw  cold  outside  air  into  the  drying 
chamber  through  leaks  and  has  also  generally  resulted  in  less  even 
air  distribution  over  the  trays.  The  second  system  keeps  the  drying 
chamber  under  slight  pressure  so  that  any  air  leakage  will  be  outward 
but  may  draw  flue  gases  from  leaks  in  the  heating  system.  A  modifica- 
tion of  the  first  system  by  placing  the  fan  in  the  heating  chamber 
and  separating  the  fan  intake  from  the  drying  chamber  by  dampers 
tends  to  eliminate  the  above  objections  by  keeping  both  heating  and 
drying  chambers  under  slight  pressure.  An  advantage  of  the  second 
system  lies  in  the  fact  that  when  the  air  is  drawn  from  the  heating 
chamber  and  blown  into  the  drying  chamber  the  churning  action  of  the 
fan  insures  a  uniform  air  temperature,  and  when  the  air  enters  the 
drying  chamber  under  pressure,  it  is  possible  to  distribute  it  uniformly 
over  the  trays  of  prunes  by  proper  placing  of  the  fan  discharge  and, 
if  necessary,  by  the  judicious  use  of  baffles. 

The  air  pressure  produced  by  a  fan  is  divided  into  velocity  pressure 
and  static  pressure,  the  sum  of  which  equals  the  total  pressure.  Static 
pressure  may  be  defined  as  the  pressure  required  to  overcome  the 
frictional  resistance  to  the  passage  of  air  between  trays  or  through 
ducts,  and  may  be  measured  in  inches  of  water  column  by  an  instru- 
ment known  as  a  pitot  tube.  The  static  pressure  in  dehydraters  is 
usually  found  to  be  between  1  and  2  inches.  In  good  average  practice 
the  static  pressure  is  about  1.5  inches.  Long,  narrow  and  crooked 
passages  or  obstructions  increase  static  pressure  and  consequently 
decrease  the  volume  of  air  passing.  In  order  that  a  fan  operating 
at  a  given  speed  may  deliver  the  greatest  possible  volume  of  air, 
it  is  essential  that  all  air  passages  used  for  heating,  drying  and 
recirculating  be  as  short  and  straight  as  possible.  No  point  in  the 
entire  system  should  have  a  free  cross  sectional  area  less  than  that 
between  the  trays  and  preferably  large  enough  to  avoid  an  air  velocity 
greater  than  1000  lineal  feet  per  minute.  The  aggregate  area  of  the 
air  passages  between  the  ends  of  the  trays  should  be  about  60  per  cent 
of  the  total  cross  sectional  area  of  the  drying  chamber. 

Air  Distribution. — In  order  to  secure  the  maximum  drying 
efficiency  of  the  air,  the  stacks  of  trays  should  as  nearly  as  possible 


24  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION 

fill  the  entire  cross  section  area  of  the  drying  chamber,  leaving  barely 
sufficient  clearance  for  movement  of  cars  as  illustrated  in  Fig.  5. 
Flexible  baffles,  commonly  made  of  discarded  canvas  belting  or  hose, 
are  advantageously  used  to  prevent  excessive  flow  of  air  over  the  top 
trays,  along  the  walls  and  under  the  cars.  In  short,  every  effort  should 
be  made  to  cause  all  the  air  to  flow  between  the  trays. 

The  free  air  space  between  the  ends  of  the  trays  usually  varies 
from  one  to  two  inches  in  height,  preferably  nearly  two  inches.  If 
these  air  spaces  are  so  narrow  as  to  materially  restrict  the  air  flow, 
drying  will  be  slow  and  uneven.  On  the  other  hand,  if  unnecessarily 
wide,  more  rapid  drying  will  not  compensate  for  the  decreased  holding 
capacity  of  the  dehydrater. 


Fig.  5. — Placing  a  car  of  prunes  in  a  tunnel  dehydrater.     (Note  how  closely  the 
stacks  of  trays  fit  the  tunnel.) 

Air  Measurement. — A  simple  method  of  measuring  the  air  flow 
in  dehydraters  is  by  the  use  of  an  anemometer  which  shows  the 
distance  in  feet  which  air  moves.  By  noting  the  distance  air  moves 
in  one  minute,  the  velocity  is  obtained  and  by  multiplying  this  by 
the  area  of  the  opening  measured,  the  volume  of  air  in  cubic  feet  per 
minute  is  obtained.  All  modern  air-blast  dehydraters  show  an  average 
air  velocity  between  trays  of  500  lineal  feet  per  minute  or  over,  gen- 
erally 600  to  700.  Velocities  below  500  feet  are  generally  associated 
with  slow  and  uneven  drying  while  velocities  in  excess  of  1000  feet 
are  not  economically  practicable. 

Power  for  Fans. — Fans,  and  burners,  require  about  1%  horse 
power  for  each  fresh  ton  of  prunes  dried  per  24  hours.  Electricity 
is  the  most  convenient  and  economical  source  of  power.  A  metal 
link  chain  is  an  efficient  fan  drive  although  more  expensive  than 
endless  water  proof  leather  belts  which  have  also  given  excellent  results 
in  dehydraters.  Eubber  fabric  belts  have  been  found  less  efficient  and 
shorter  lived  in  dehydrater  work. 


BULL.  404]  THE   DEHYDRATION   OF   PRUNES  25 


HUMIDITY   CONSIDERATIONS 

Definition  of  Humidity. — The  water  vapor  present  in  air  is  com- 
monly expressed  as  relative  humidity,  or  the  percentage  of  the  weight 
of  water  vapor  in  a  given  space  to  the  weight  of  water  vapor  which 
the  same  space  at  the  same  temperature  could  hold  if  it  were  saturated 
with  moisture.  Saturated  air  has  a  relative  humidity  of  100  per  cent 
and  absolutely  dry  air  of  0  per  cent. 

Measurement  of  Humidity. — Relative  humidity  is  determined  by 
the  comparative  readings  of  two  thermometers,  one  having  a  dry  bulb 
and  the  other  having  its  bulb  closely  covered  by  a  clean  wick  kept 
moist  by  distilled  water.  These  thermometers  are  placed  together  in 
the  direct  air  flow  of  the  drying  chamber,  usually  at  the  point  of 
highest  temperature.  The  dry  bulb  thermometer  indicates  the  tem- 
perature and  the  reading  of  the  wet  bulb  thermometer  will  be  lower 
because  of  the  cooling  effect  of  the  evaporation  of  moisture  from  the 
moist  wick  surrounding  the  bulb.  The  lower  the  moisture  content  of 
the  air,  the  more  rapid  will  be  evaporation  and  consequently  the  lower 
the  reading  of  the  wet  bulb  thermometer.  Use  has  been  made  of  this 
simple  principle  in  preparing  charts  such  as  the  one  in  Fig.  6,  giving 
the  relative  humidity  of  the  air  for  any  combination  of  wet  and  dry 
bulb  temperatures. 

Effect  of  Temperature  on  Humidity. — The  moisture  holding 
capacity  of  air  approximately  doubles  for  every  27°  rise  in  tempera- 
ture, or,  in  other  words,  the  relative  humidity  of  air  is  halved  when 
its  temperature  is  raised  27°.  For  example,  if  a  given  weight  of  air 
outside  a  dehydrater  had  a  temperature  of  57°  and  a  relative  humidity 
of  100  per  cent,  as  might  be  the  case  on  a  rainy  or  foggy  day,  and  if 
this  air  were  drawn  into  the  dehydrater  and  heated  to  165°  F.,  it 
would  have  a  relative  humidity  of  only  about  6  per  cent,  or,  in  other 
words,  the  same  weight  of  air  could  hold  16  times  as  much  water  as  it 
originally  held.  When  air  is  considered  in  terms  of  volume,  instead  of 
weight,  these  figures  are  modified  because  of  the  expansion  of  the 
air  on  heating  but  they  serve  as  a  simple  explanation  of  why  dehy- 
draters  continue  drying  independently  of  the  humidity  of  the 
external  air. 

Recirculation. — If  a  dehydrater  were  hermetically  sealed  to  prevent 
interchange  of  air  with  the  outside  and  the  air  within  continuously 
recirculated  and  reheated  it  would  soon  reach  saturation  and  drying 
would  cease.  If,  on  the  other  hand,  air  were  drawn  into  a  dehydrater, 
heated  and  then  discharged  after  passing  over  the  fruit  only  once, 
an  excessive  amount  of  heat  would  be  wasted  with  the  exhaust  air. 


fiuui     8  VcVipeW  E-ATUR&     § 


Fig.  6. — Chart  for  determining  humidity  from  wet  and  dry  bulb  temperatures. 
(Drawn  by  G.  B.  Eidley.) 


BULL.  404]  The   DEHYDRATION    OF   PRUNES  27 

Since  the  first  of  these  conditions  is  impossible  for  drying  and  the 
second  wasteful  of  fuel,  it  is  obvious  that  between  the  two  is  a  condition 
which  permits  comparatively  rapid  drying  and  at  the  same  time  gives 
optimum  fuel  efficiency.  The  volume  of  air  required  for  absorption 
of  the  moisture  evaporated  from  the  fruit  is  on  the  average  only  y7  to 
%  as  much  as  the  volume  of  air  necessary  to  convey  the  heat  required 
for  evaporation.  Recirculation  means  the  re-use  of  a  portion  of  the 
warm  exhaust  air  to  which  is  added  sufficient  fresh  air  to  make  the 
resultant  mixture  after  reheating  contain  no  more  water  vapor  than 
it  did  before  passing  over  the  fruit  previously.  If  this  be  done,  the 
humidity  of  the  air  entering  the  drying  chamber  can  be  maintained 
at  any  desired  per  cent  and  drying  will  progress  steadily  with  the 
minimum  loss  of  heat  in  the  exhaust  air.  Because  of  their  length  and 
complexity,  calculations  on  control  of  recirculation  are  omitted  from 
this  bulletin  but  are  available  elsewhere  (see  reference  No.  2,  page  47). 
A  little  experience  will  enable  any  operator  to  so  adjust  the  exhaust 
air  and  fresh  air  intake  dampers  of  his  dehydrater  as  to  maintain  the 
relative  humidity  of  the  air  within  the  dehydrater  at  that  per  cent 
which  experience  shows  him  to  give  the  most  rapid  drying  consistent 
with  reasonable  fuel  economy.  Practical  experience  has  shown  that 
partial  recirculation  decreases  fuel  consumption  at  least  50  per  cent 
without  decrease  in  the  rate  of  drying  and  all  commercially  built 
dehydraters  now  use  this  system. 


CALCULATION  OF  DEHYDRATER  REQUIREMENTS 

The  following  typical  example  of  dehydrater  requirements  is 
presented  as  a  guide  in  determining  the  adequacy  of  a  dehydrater  and 
its  essential  parts.  Let  it  be  assumed  that  a  certain  prune  grower  must 
have  a  dehydrater  capable  of  dehydrating  10  fresh  tons  in  a  day  of 
24  hours  in  order  to  accommodate  the  peak  load  of  a  normal  harvest 
and  that  these  prunes  will  have  an  average  drying  ratio  of  2.5  to  1. 

Assuming  an  average  drying  time  of  24  hours,  the  dehydrater  must 
have  a  holding  capacity  of  20,000  pounds  of  fresh  prunes.  If  the  trays 
have  an  average  load  of  3.5  pounds  per  square  foot,  5714  square  feet 
of  tray  area  must  be  provided  for  in  the  dehydrater.  If  two  stacks 
of  3'  X  8'  trays,  25  high,  are  used  to  the  truck,  the  capacity  should 
be  5  such  trucks,  if  3'  X  3'  trays,  13  trucks. 

To  dehydrate  20,000  pounds  of  prunes  with  a  drying  ratio  of  2.5 
to  1  in  24  hours,  necessitates  the  evaporation  of  12,000  pounds  of  water 
or  500  pounds  an  hour.  Assuming  an  overall  fuel  efficiency  of  45  per 
cent,  2444  B.T.U.  per  lb.  of  water  evaporated  will  be  required   (see 


28  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

page  18)  or  1,222,000  B.T.U.  per  hour.  Assuming  a  heat  value  of 
142,000  B.T.U.  for  a  gallon  of  oil,  8.6  gallons  of  oil  must  be  burned. 
Consequently  a  burner  of  not  less  than  9  gallons  capacity  an  hour 
should  be  provided.  For  capacity  of  oil  burners  reference  should  be 
had  to  the  catalogs  of  firms  manufacturing  oil  burners  for  dehydraters. 

If  the  direct  radiation  system  be  used,  it  is  essential  that  the  total 
surface  area  of  furnace  and  flues  be  adequate  to  radiate  the  required 
heat.  The  surface  required  varies  greatly  with  the  nature  of  the 
furnace  and  flues  and  no  exact  figure  can  be  given.  With  the 
common  steel  radiating  furnace  and  flues,  from  400  to  500  square  feet 
is  usual  for  a  plant  of  this  size. 

An  evaporation  of  500  pounds  of  water  an  hour  is  equal  to  8.33 
pounds  a  minute.  By  reference  to  the  figures  on  page  21  it  can  be 
seen  that  if  the  air  has  a  temperature  drop  of  35°  and  an  evaporating 
efficiency  of  75  per  cent  in  passing  through  the  drying  chamber,  each 
pound  of  water  to  be  evaporated  each  minute  requires  the  passage  of 
2704  cubic  feet  of  air.  Consequently,  an  evaporation  of  8.33  pounds 
of  water  (one  gallon)  a  minute  will  require  22,532  cubic  feet  of  air  a 
minute. 

If  this  entire  volume  of  air  is  to  be  delivered  by  a  single  fan,  a 
multivane  fan  will  be  best.  In  some  types  of  dehydraters,  a  number 
of  smaller  fans,  usually  of  the  disc  type,  are  used  to  give  the  total 
air  flow  required.  No  portion  of  any  air  passage  should  have  a  cross 
section  area  less  than  that  of  the  fan  discharge  connected  to  it.  If 
the  total  air  flow  is  delivered  by  a  single  fan,  no  part  of  the  entire  air 
system  should  have  an  area  of  less  than  22.5  square  feet,  except  at 
the  fan.  The  total  free  area  between  the  ends  of  the  trays  (see  page 
24)  should  be  about  30  square  feet,  which  will  give  an  air  velocity  of 
750  lineal  feet  a  minute  between  trays. 

Most  fan  manufacturers  furnish  tables  of  performance  which  show 
for  each  size  of  fan  the  volume  of  air  delivered  at  a  given  speed  and 
a  given  static  pressure  and  the  horsepower  required.  By  reference  to 
such  tables  and  to  price  lists,  selection  can  be  made  of  the  fan  which 
will  most  economically  deliver  the  volume  of  air  required.  The  static 
pressure  in  dehydraters  usually  varies  from  1  to  2  inches.  If  the 
dehydrater  is  constructed  with  attention  to  the  principles  regarding 
the  free  area  and  construction  of  air  passages,  an  average  static 
pressure  of  1%  inches  is  ordinarily  used  in  determining  fan  capacities. 

The  foregoing  paragraphs  have  given  briefly  the  most  important 
factors  concerned  in  the  construction  of  any  dehydrater,  namely,  its 
holding  capacity  for  prunes  and  the  amount  of  heat  and  air  required 
to  dry  the  prunes  in  a  given  time.     The  figures  used  in  the  example 


BULL.  404]  THE   DEHYDRATION   OF  PRUNES  29 

are  purposely  conservative  and  many  dehydraters  show  greater  effi- 
ciency. For  instance,  if  the  drying  ratio  is  less  than  2%  :  1  or  the  fuel 
efficiency  more  than  45  per  cent,  correspondingly  less  heat  and  air  will 
be  required  to  dry  the  same  weight  of  prunes  in  the  same  time. 


SELECTION  OF  A  DEHYDRATER 

The  following  three  courses  are  open  to  growers  who  wish  to  install 
a  dehydrater: 

1.  Purchase  of  a  standard  commercially  built  dehydrater. 

2.  Construction  of  a  dehydrater  from  plans  furnished  by  a  dehy- 
drater manufacturer  or  engineer. 

3.  Construction  of  a  dehydrater  from  an  original  design. 

Most  growers  find  it  simpler,  safer  and  just  as  inexpensive  to  buy 
a  standard  commercial  dehydrater  as  to  design  or  build  their  own 
plant.  The  few  growers  who  have  been  successful  in  building  their 
own  dehydraters  are  usually  men  with  previous  experience  in  con- 
struction or  engineering  work.  Without  some  technical  training  or 
experience  in  such  matters,  it  is  inadvisable  for  a  grower  to  attempt  the 
construction  of  a  dehydrater.  Some  growers  have  the  impression  that 
dehydrater  manufacturers  charge  excessive  profits  but  such  is  not 
generally  the  case.  The  quantity  purchase  of  materials  and  equipment 
at  low  prices  and  the  employment  of  experienced  mechanics  enables 
large  dehydrater  manufacturers  to  sell  plants  at  a  profit  for  a  price 
little,  if  at  all,  greater  than  that  for  which  a  grower  could  build  a 
single  plant. 

There  have  been  several  instances  of  successful  dehydraters  built 
by  growers  from  plans  obtained  from  dehydration  engineers.  If 
such  plans  have  been  demonstrated  to  give  an  efficient  dehydrater, 
some  saving  can  often  be  effected  by  this  scheme.  In  this  connection, 
it  should  be  mentioned  that  the  University  of  California  does  not  fur- 
nish plans  for  dehydraters. 

Descriptions  of  the  leading  types  of  commercial  dehydraters  some 
of  which  are  illustrated  in  Figs.  7,  8,  9  and  10  are  not  included  because 
they  are  preferably  obtained  from  pamphlets  issued  by  the  manufac- 
turers or  better  by  inspecting  the  dehydraters.  Growers  are  strongly 
urged  to  confine  their  selection  of  a  dehydrater  to  types  which  have 
already  been  built  and  operated  so  as  to  demonstrate  their  capacity 
and  efficiency.  Persons  wishing  to  manufacture  and  sell  dehydraters 
should  not  expect  growers  to  invest  in  a  machine  until  the  prospective 
manufacturer  has  demonstrated  his  claims  by  operating  such  a  dehy- 
drater through  a  prune  season. 


30 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


DEHYDRATER  MANUFACTURERS 

The  following  list  includes  the  names  of  all  persons  or  firms  that 
sold  dehydraters  for  primes  during  1925.  Persons  formerly  in  this 
business  but  not  active  therein  at  present  or  persons  seeking  to  sell 
dehydraters  but  who  have  not  yet  succeeded  are  not  included : 

Chapman  Dehydrater  Company,  Bare  Building,  Modesto. 

W.  W.  Cozzens,  10  Broadway,  San  Jose. 

Knipschild  Dehydrater  Company,  St.  Helena. 

Oliver  Dehydrater  Company,  Lincoln  Avenue  and  Moorpark  St., 
San  Jose. 

G.  B.  Ridley,  255  California  Street,  San  Francisco. 

Progressive  Dehydrater  Company,  340  Seventh  St.,  San  Francisco. 

Puccinelli  Dehydrater  Company,  Los  Gatos. 


Fig.  7. — Kemoving  a  tray  of  prunes  from  an  Oliver  Natural  Draft  Dehydrater. 


PATENT  SITUATION 

Any  grower  designing  his  own  dehydrater  should  do  two  things 
before  beginning  construction,  first,  have  the  plans  and  specifications 
checked  by  a  competent  authority  to  ascertain  if  they  will  accomplish 
the  desired  result  and,  second,  ascertain  if  the  design  infringes  any 
dehydrater  patent.  While  there  is  no  basic  patent  covering  the 
dehydration  of  prunes,  most  types  of  dehydraters  in  use  are  protected 
in  whole,  or  in  part,  by  patents.     These  patents  are  of  the  type  com- 


3 


I 

u 

5 

3 
p 


p 

CD 


TO 


fc/O 
o 

(-< 

to 

1=1 


to 


Bull.  404] 


THE   DEHYDRATION   OF   PRUNES 


33 


3 


I 
a 

go* 

CfQ 
•-* 
P 

5 

3 

p 


o 

cs 

CD 

a 

is- 
p 

B 

p 


S3* 


34 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


monly  referred  to  as  "construction  patents"  and  care  should  be  taken 
to  avoid  legal  entanglements  occasioned  by  unauthorized  duplication 
of  patented  features.  One  should  not  be  misled  by  the  common 
misconception  that  a  grower  building  a  dehydrater  exclusively  for 
his  own  use  is  exempt  from  patent  infringement  claims.  So  far  no 
damage  suits  have  been  brought  for  infringement  of  dehydrater 
patents  and  consequently  the  situation  remains  uncertain  pending 
clarification  through  judicial  decision. 

Dehydrater  Patents.- — Of  the  many  hundreds  of  patents  granted  on 
dehydrating  equipment,  the  following  have  been  selected  as  of  the 
greatest  present  importance  in  avoiding  possible  infringements  when 
new  types  of  dehydraters  are  built  and  used. 


Number 

Filed 

Granted 

Issued  to 

1,461,224 

Dec.   13,  1919 

July    10,  1923 

J.  W.   Pearson   (associate  of 
Ridley) 

G.  B. 

1,413,125 

Jan.    15,  1920 

Apr.    18,  1922 

Claude    Rees     (Progressive 
drater  Co.) 

Dehy- 

1,404,369 

May     1,  1920 

Jan.    24,  1922 

F.  C.  Chapman 

1,464,338 

June   29,  1921 

Aug.     7,  1923 

R.  L.  Puccinelli 

1,528,223 

Feb.   21,  1922 

Mar.     3,  1925 

C.  C.  Moore 

1,532,303 

Dec.     4,  1922 

Apr.      7,  1925 

W.  W.  Cozzens 

1,543,947 

Aug.   13,  1923 

June   30,  1925 

C.  C.  Moore 

A  copy  of  any  patent  can  be  obtained  by  sending  ten  cents  in  coin 
to  the  U.  S.  Commissioner  of  Patents,  Washington,  D.C.  A  patent 
attorney  should  be  consulted  on  questions  concerning  infringement. 


SOME  CONSTRUCTION  PRINCIPLES 

While  it  is  not  the  purpose  of  this  bulletin  to  enter  into  a 
discussion  of  the  relative  merits  of  the  several  types  of  dehydraters, 
the  following  suggestions  will  be  found  helpful : 

In  order  to  minimize  fixed  charges  the  capacity  of  the  dehydrater 
should  not  be  larger  than  is  necessary  to  dry  the  expected  tonnage. 
First  cost  is  a  most  important  factor  in  fixed  charges  and  it  is  often 
sound  economy  to  sacrifice  a  certain  amount  in  operating  efficiency 
in  order  to  obtain  a  greater  saving  on  the  investment  and  fixed  charges. 

Custom  Dehydration. — Many  dehydrater  owners  have  engaged  in 
custom-drying  prunes  for  neighbors.  The  increased  seasonal  tonnage 
reduces  the  fixed  charges  per  ton  and  results  in  a  profit  to  the  operator. 
The  custom  charge  for  dipping  and  dehydrating  prunes  has  usually 
varied  from  $10  to  $15  a  fresh  ton,  averaging  $12.50.    Comparing  this 


BULL.  404]  TIIE   DEHYDRATION    OF   PRUNES  35 

figure  with  an  average  total  cost  of  dehydrating  of  $8  a  ton  it  can 
be  seen  that  custom-drying  is  a  profitable  venture  during  the  few 
weeks  prunes  are  available  for  drying,  provided  the  dehydrater  is 
operated  at  normal  capacity. 

Community  Dehydraters. — Growers  with  small  acreages  and 
limited  finances  will  do  well  to  consider  the  advantages  of  a  community 
dehydrater.  In  general,  the  larger  the  dehydrater  the  lower  the 
investment  and  the  less  the  total  operating  costs  per  ton  will  be. 
However,  in  order  for  a  community  dehydrater  to  be  successful,  the 
owners  should  have  the  true  cooperative  spirit  and  the  management, 
both  financial  and  operating,  should  be  carefully  planned  to  accom- 
modate the  crops  of  the  several  owners  in  an  efficient,  impartial  manner. 

Fire-proof  construction  is  preferred  for  dehydraters  not  only 
because  it  precludes  the  loss  of  the  plant  by  fire  which  is  especially 
like]}*  to  occur  when  most  needed  during  the  prune  harvest,  but  it 
eliminates  or  very  greatly  reduces  the  high  insurance  premiums  on 
dehydraters  built  of  combustible  materials  such  as  wood.  Hollow 
walls,  usually  built  of  tile  or  concrete  blocks,  are  most  common,  the 
dead  air  space  serving  to  reduce  heat  radiation  losses.  Double  walls 
of  wood,  sheet  metal,  asbestos  or  other  building  boards,  have  also  been 
successfully  used  and  are  less  expensive  to  erect.  Solid  concrete  is 
no  longer  used  much  because  it  is  expensive  construction  and  does  not 
retain  heat  so  well  as  hollow  walls.  In  any  case,  the. construction  should 
be  compact  and  tight  to  prevent  leakage  of  heated  air.  The  doors 
should  be  especially  substantial  and  tight  fitting. 

As  explained  on  pages  27-29  the  vital  parts  of  the  dehydrater 
such  as  oil  burner,  furnace,  flues,  fan,  motor,  etc.,  should  be  of  adequate 
size,  strongly  constructed  and  firmly  mounted. 

Accurate  wet  and  dry  bulb  thermometers  should  be  provided. 
Recording  thermometers,  while  comparatively  expensive,  are  valuable 
in  furnishing  a  permanent  record  of  the  temperature  of  the  dehydrater 
at  all  times,  both  wet  and  dry  bulbs  if  desired.  The  charts  of  such 
thermometers  are  mounted  once  a  day  in  a  locked  case  and  thereby 
serve  as  a  check  on  the  operator  when  the  owner  is  absent. 

Cars  and  Trays. — The  minimum  number  of  cars  and  trays  required 
for  dipping  and  drying  is  50  per  cent  in  excess  of  the  holding  capacity 
of  the  dehydrater.  Experienced  operators  state  that  100  per  cent 
excess  is  required  to  obtain  maximum  flexibility  and  efficiency. 

Solid  bottom  field  trays,  with  sides  or  ends  reconstructed  so  as  to 
permit  adequate  air  flow  between  the  trays  when  stacked,  are  commonly 
used.  However,  if  new  trays  are  to  be  provided  special  dehydrater 
trays  with  slat  bottoms  will  generally  give  slightly  more  rapid  and 


36 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


more  even  drying  than  solid  bottom  trays.  Screen  trays  are  more 
expensive  and  after  being  used  for  some  time,  the  sagging  of  the 
screen  causes  uneven  drying. 

For  ease  in  handling  the  heavy  loads  of  trays,  the  cars  should  be 
equipped  with  roller  bearings  and  in  large  dehydraters  a  winch  and 
cable  is  necessary  for  moving  the  cars  through  the  dehydrater. 


Fig.  11. — Suggested  arrangement  for  a  prune  dehydrater. 


ARRANGEMENT    OF    EQUIPMENT 

Since  most  of  the  labor  in  dehydration  is  required  before  and 
after  drying,  rather  than  during  drying,  it  is  obvious  that  careful 
consideration  of  these  operations  will  result  in  maximum  efficiency 
and  minimum  cost  of  labor.  The  ideal  arrangement  of  the  entire 
plant  is  that  in  which  the  prunes  move  from  the  receiving  platform 
to  the  storage  bins  in  a  continuous,  unimpeded  circuit  by  the  shortest 
practical  route,  the  trucks  of  emptied  trays  being  conveniently 
returned  to  the  loading  point.  The  compactness  of  the  plant  is  also 
of  importance  in  securing  a  neat  appearance  and  economy  of  ground 
space  as  well  as  a  saving  in  the  construction  of  roof  area,  tracks,  etc. 
It  is  impossible  to  present  a  plan  which  will  exactly  fit  all  cases,  but 
the  plan  presented  in  Fig.  11  will  fit  most  installations  and  is  suscep- 
tible of  adaptation  to  any  dehydrater  now  in  use.  The  main  features 
of  this  plan  are: 


BULL.  404]  THE   DEHYDRATION   OF   PRUNES  37 

1.  All  the  operations  of  receiving,  dipping,  grading,  loading  and 
unloading  trays  are  concentrated  in  one  location  so  as  to  always  be 
under  the  direct  observation  of  the  person  in  charge. 

2.  The  path  of  the  prunes  is  such  that  they  constantly  move 
forward  without  retracing  of  routes,  thereby  preventing  interference 
of  the  cars  of  fresh  fruit  with  those  holding  dried  fruit. 

3.  The  emptied  trays  are  available  for  reloading  at  a  point  close 
to  the  dipper  discharge,  thereby  avoiding  extra  handling  of  empty 
cars  and  trays. 


OPERATION  OF  DEHYDRATERS 

The  methods  of  harvesting,  dipping  and  traying  prunes  are  pri- 
marily the  same  whether  the  prunes  are  to  be  sun-dried  or  dehydrated. 
These  have  already  been  described  in  Bulletin  388,  "The  Principles 
and  Practice  of  Sun-Drying  Fruits, ' '  and  need  not  be  repeated  here. 
The  operation  of  dehydraters  is  best  learned  by  experience,  supple- 
mented by  visits  to  efficiently  operated  plants.  No  one  system  will 
fit  all  types  of  dehydraters  or  all  varieties  of  prunes.  However,  in 
order  that  operators  may  be  guided  in  the  right  direction,  the  following 
principles  of  operating  prune  dehydraters  are  presented : 

dipping 

Lye  dipping  of  prunes  is  as  essential  in  dehydration  as  in  sun 
drying  and  should  be  followed  by  rinsing  in  clear  water,  preferably 
with  sprays.  The  washing  off  of  the  waxy  bloom,  as  well  as  dirt, 
and  the  checking  of  the  skin  permits  the  prunes  to  be  dehydrated 
more  rapidly  and  evenly  than  if  undipped  and  gives  clean  fruit. 
If  lye  dipping  is  property  controlled  the  use  of  pricker  boards  is  of 
doubtful  value  except  on  very  thick  skinned  prunes,  as  "bloaters" 
rarely  occur  in  dehydration.  For  all  but  the  smallest  plants,  rotary 
drum  dippers  are  preferred  because  of  their  continuous  operation 
and  economical  use  of  labor.  Operators  of  some  natural  draft  dryers 
report  no  difference  in  the  drying  time  of  dipped  and  undipped 
prunes.  Investigations  have  shown  this  to  be  due  to  the  limited  ability 
of  such  dryers  to  evaporate  water,  which  enabled  undipped  prunes 
to  give  up  their  moisture  as  rapidly  as  the  dryer  could  remove  it. 
This  is  not  the  case  in  air-blast  dehydraters.  Over-dipping  should  be 
avoided  because  it  causes  the  prunes  to  bleed  and  drip,  giving  sticky 
fruit  and  trays.  Tender  skinned  varieties,  such  as  the  Imperial,  are 
often  dipped  in  plain  hot  water  to  prevent  excessive  cracking  of  the 
skins. 


38  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


GREEN  GRADING 

It  is  obvious  tha,t  the  more  even  is  the  size  of  fresh  prunes  on  a 
given  car  of  trays,  the  more  evenly  dried  the  prunes  will  be  when 
removed  from  the  dehydrater.  It  has  sometimes  been  claimed  that 
conditions  in  a  dehydrater,  especially  humidity,  could  be  so  controlled 
as  to  cause  all  prunes  to  dry  evenly  to  the  same  moisture  content 
regardless  of  their  size  or  composition.  This  is  not  true  because  when 
a  prune  is  placed  in  a  dehydrater  it  continues  to  give  up  moisture  to 
the  air  until  its  remaining  moisture  content  comes  into  equilibrium 
with  the  moisture  content  of  that  air.  Unfortunately,  the  final  mois- 
ture content  of  a  prune  which  would  reach  equilibrium  with  an  air 
humidity  which  could  be  economically  maintained  in  a  dehydrater  is 
much  below  the  moisture  content  of  the  prune  when  considered  suffi- 
ciently dry  to  be  removed  from  the  dehydrater.  Therefore,  if  all 
prunes  were  allowed  to  reach  this  equilibrium,  an  unnecessary  loss  in 
weight  of  dried  fruit  would  result.  Consequently,  if  large  or  plump 
prunes  are  placed  on  the  same  car  of  trays  with  small  or  partially 
dried  prunes,  it  is  impossible  for  all  such  prunes  to  have  the  same 
moisture  content  when  removed  from  the  dehydrater.  In  order  to 
minimize  this  inequality  and  to  obtain  as  uniform  a  moisture  content 
in  the  dried  product  as  possible,  it  is  advisable  to  grade  the  fresh 
prunes  into  two  or  more  sizes. 

In  addition  to  promoting  even  dryness,  green  grading  has  another 
important  advantage  in  increasing  the  drying  capacity  of  a  dehydrater. 
For  example,  let  it  be  assumed  that  a  certain  dehydrater  has  an  average 
drying  capacity  of  10  cars  of  ungraded  fresh  prunes  in  24  hours  or  a 
total  of  240  car  hours.  Now  if  these  prunes  were  green  graded  into 
half  "number  ones"  and  half  "number  twos,"  the  dehydrater  could 
dry  these  prunes  at  the  rate  of  5  cars  of  "ones"  in  24  hours  and  5  cars 
of  "twos"  in  say  20  hours,  or  a  total  of  220  car  hours.  In  other  words, 
the  average  drying  time  would  be  reduced  from  24  to  22  hours  as  a 
result  of  the  faster  drying  of  the  "twos"  separated  by  green  grading. 
Therefore,  the  daily  capacity  of  the  dehydrater  would  be  increased  by 
8.33  per  cent  or  1666  pounds  of  fresh  prunes,  nearly  one  car  load. 
This  advantage  may  be  partially  counterbalanced  if  it  is  found  impos- 
sible to  safely  spread  as  many  pounds  of  "twos"  as  of  "ones"  on  a 
tray.  In  plants  having  more  than  one  unit  or  track,  the  different 
sizes  of  prunes  are  preferably  dried  in  separate  lines  but  even  in 
single  unit  plants  it  is  possible  to  realize  the  advantages  of  green 
grading  by  proper  judgment  in  entering  the  cars  of  "twos,"  usually 
behind  the  "ones,"    In  the  latter  case,  often  two  cars,  one  each  of 


BULL.  404]  THE   DEHYDRATION   OF  PRUNES  39 

"ones"  and  "twos"  can  be  removed  as  dry  at  the  same  time,  making 
room  for  two  cars  of  fresh  frnit  instead  of  only  one.  The  same 
principle  applies  to  natural  draft  dehydraters  also. 

Taking  all  factors  into  consideration,  observation  indicates  the 
desirability  of  green  grading  in  all  but  the  smallest  plants. 

traying 

For  continuous  spreading  of  dipped  and  graded  prunes  and  mini- 
mum handling  of  trays,  the  diagonal  discharge  illustrated  in  Fig.  11. 
has  given  satisfaction.  A  continuous  stream  of  empty  trays  on  a  roller 
conveyor  is  passed  under  the  ends  of  the  grader,  the  diagonal  discharge 
being  so  regulated  that  the  prunes  are  evenly  distributed  over  the 
entire  tray  with  little  hand  spreading.  To  facilitate  removal  of  the 
dried  prunes  from  the  trays  without  injury  to  either  fruit  or  trays 
caused  by  sticking,  it  is  necessary  to  keep  the  trays  clean.  It  is  often 
necessary  to  wash  them  several  times  in  the  season. 

The  filled  trays  are  stacked  on  trucks  standing  on  a  track  close  at 
hand.  Where  two  grades  are  being  loaded  separately,  greater  flexi- 
bility in  handling  will  be  obtained  by  using  two  parallel  loading 
tracks.  One  of  these  may  run  under  the  grader  or  tray  conveyor 
because  of  the  low  bed  of  the  empty  dehydrater  trucks,  the  trays  from 
the  emptied  truck  having  been  passed  over  the  conveyor  for  loading 
the  car  ahead.  The  conveyor  for  each  size  of  prunes  is  supplied  with 
trays  from  the  corresponding  track.  In  most  plants,  dipping  is  done 
only  on  the  day  shift  while  dehydrating  is  carried  on  continuously. 
In  order  that  the  dehydrater  may  be  supplied  with  prunes  all  night 
it  is  customary  to  have  sufficient  loaded  trucks  by  evening  to  keep  the 
dehydrater  filled  until  the  next  morning.  For  these  trucks,  track 
space  equivalent  to  over  half  the  capacity  of  the  plant  must  be 
provided. 

Stackers. — In  order  that  the  high  stacks  of  trays  will  pass  through 
a  close  fitting  drying  chamber  without  being  broken  or  knocked  off 
or  becoming  jammed  so  as  to  impede  the  movement  of  cars,  it  is 
important  that  the  stacks  be  vertical  and  centered  on  the  car.  Guide 
posts  erected  along  side  the  loading  tracks  help  materially  in  securing 
a  proper  stack  without  particular  attention  on  the  part  of  the  men 
doing  the  stacking. 

Automatic  stackers  are  in  use  at  several  large  dehydraters.  These 
machines  consist  of  a  steel  frame  work  supporting  two  sets  of  endless 
chains  operating  on  sprocket  wheels  and  turned  by  a  small  motor. 
The  chains  are  provided  with  pairs  of  flexible  metal  fingers  at  regular 


40  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

intervals  which,  when  a  loaded  tray  is  rolled  into  the  stacker  from 
the  dipper,  pick  up  the  tray,  carry  it  up,  over  and  deposit  it  down 
on  the  car  being  filled.  By  means  of  angle  iron  guides,  a  perfect 
stack  is  obtained  without  attention,  except  to  periodically  replace 
loaded  with  empty  cars.  The  construction  is  adaptable  to  any  size 
or  number  of  trays. 

Observations  have  shown  that  the  automatic  stacker  eliminates 
two  men  from  a  five  man  dipping  and  loading  crew.  Since  the  fixed 
charges  on  a  stacker  are  approximately  equal  to  the  wages  of  one  man 
for  the  normal  prune  season,  its  use  is  in  line  with  economy. 

Bumpers. — The  relative  heights  of  cars  and  trays  are  often  suffi- 
ciently irregular  so  that  the  free  air  spaces  between  trays  on  adjacent 
cars  do  not  coincide,  in  which  case  the  air  flow  between  trays  may  be 
impeded.  Where  such  a  condition  exists,  it  is  necessary  to  place 
bumpers  on  both  ends  of  each  car  so  as  to  separate  the  stacks  of  trays 
by  two  to  four  inches  and  thereby  permit  unimpeded  air  flow. 


TEMPERATURE 

\ems  of  Dehydration. — There  are   four   possible   systems  by 
which  dehydraters  may  be  operated  with  respect  to  temperature. 

1.  Counter  current,  in  which  the  fruit  enters  at  one  end  of  the 
dehydrater  at  a  relatively  low  temperature  and  is  advanced  inter- 
mittently to  the  other  end  for  finishing  at  the  maximum  temperature. 

2.  Parallel  current,  opposite  of  counter  current,  the  fruit  entering 
at  the  highest  temperature  and  finishing  at  the  lowest. 

3.  Combination  system,  in  which  the  maximum  temperature  is 
maintained  at  the  center  of  the  dehydrater  and  the  fruit  enters  at 
one  end  at  a  relatively  low  temperature,  passes  through  the  highest 
temperature,  while  still  only  partially  dried  and  finishes  at  the  other 
end  at  a  lower  temperature. 

4.  Constant  temperature,  in  which  the  fruit  is  subject  to  a  constant 
temperature  throughout  the  drying  period. 

The  counter  current  system  is  the  one  most  commonly  used  on 
prunes  in  California,  because  nearly  all  air-blast  dehydraters  are  of 
the  tunnel  type,  or  a  long  drying  chamber  through  which  the  cars 
pass  in  a  continuous  stream  while  the  moisture  is  evaporated  by  a 
current  of  heated  air  passing  in  the  opposite  direction.  In  the 
dehydraters  designed  by  Puccinelli,  Ridley,  Knipschild  and  others, 
the  air  current  passes  straight  through  the  drying  chamber  while  in  the 
Progressive  dehydrater  it  follows  a  helical  or  "corkscrew"  motion. 


BULL.  404]  TIIE   DEHYDRATION    OF   PRUNES  41 

The  combination  system  is  employed  in  the  Chapman  dehydrater, 
which  is  a  modification  of  the  tunnel  type,  with  the  heated  air  entering 
at  the  center,  dividing  and  passing  equally  toward  both  ends. 

The  Cozzens  dehydrater  is  the  nearest  example  to  a  constant 
temperature  dehydrater.  The  cars  enter  through  doors  along  the  side 
of  the  tunnel  and  do  not  move  progressively  through  the  tunnel. 
However,  they  are  removed  from  one  section  to  another  nearer  the 
hot  end.  This  is  advantageous  when  two  or  more  kinds  of  fruit  are 
being  dehydrated  simultaneously,  for  example,  peaches  and  prunes. 

The  parallel  current  system  has  not  proved  satisfactory  for  prunes 
and  is  not  used. 

Critical  Temperature. — It  is  a  well  known  fact  that  fruit  sugars, 
especially  levulose,  will  gradually  caramelize  and  suffer  a  loss  in  weight 
if  subjected  to  temperatures  above  160°  F.,  the  higher  the  temperature 
the  more  rapid  being  the  loss.  However,  the  sugar  in  prunes  will  not 
be  affected  by  temperatures  considerably  in  excess  of  160°  F.  so  long 
as  the  prunes  are  continuing  to  give  up  moisture  freely.  The  tempera- 
ture of  a  prune  from  which  water  is  being  evaporated  in  a  dehydrater 
approaches  that  of  a  wet  bulb  thermometer  and  is  considerably  lower 
than  the  dry  bulb  temperature  of  the  surrounding  air.  It  is  for  this 
reason  that  partially  dried  prunes  can  be  subjected  to  temperatures 
considerably  above  160°  F.,  without  loss  in  sugar,  provided  none  of 
the  prunes  are  already  so  nearly  dry  that  they  will  be  injured  by  such 
higher  temperatures.  When  a  prune  approaches  such  a  reduced 
moisture  content  that  drying  becomes  relatively  slow,  its  temperature 
will  approach  that  of  the  dry  bulb  thermometer  and  it  is  at  this  point 
that  prunes  should  be  removed  from  the  dehydrator  in  order  to 
prevent  further  drying  and  possible  injury.  For  these  reasons,  it  has 
been  found  unsafe  to  finish  prunes  at  a  temperature  above  165°  F., 
and  this  temperature  has  therefore  been  adopted  as  a  standard. 

Temperatures  above  165°  F.  have  been  occasionally  used  without 
apparent  injury  to  prunes,  although  it  is  possible  that  there  was  a 
small  undetermined  loss  in  sugar  in  such  cases.  While  temperatures 
below  165°  F.  may  produce  equally  good  dried  prunes,  their  use  is 
generally  inadvisable  because  the  slower  drying  results  in  decreased 
capacity  and  a  greater  cost  of  operation. 

The  drying  of  plump  prunes  should  not  be  commenced  at  high 
temperature  because  the  sudden  expansion  of  the  prunes  will  cause 
them  to  split  and  bleed,  resulting  in  a  loss  in  weight  and  sticky  fruit 
and  trays.  Furthermore,  the  desired  darkening  of  the  skin  is  arrested 
at  high  temperatures  and  prunes  dried  too  rapidly  at  high  temperatures 


42  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

have  been  observed  to  develop  a  reddish  brown  rather  than  a  black 
skin.  For  these  reasons  it  is  considered  good  practice  to  enter  the 
prunes  at  the  cooler  end  of  the  dehydrater,  usually  at  120°  F.  to 
140°  F.,  and  move  them  progressively  toward  the  highest  temperature 
for  finishiug. 

HUMIDITY 

The  rate  of  evaporation  from  a  free  water  surface  decreases  with 
an  increase  in  the  relative  humidity  of  the  air.  However,  the  cellular 
structure  and  syrupy  nature  of  fruit  tissues  retard  evaporation,  so  that 
under  no  condition  does  the  rate  of  evaporation  equal  that  from  a  free 
water  surface.  When  conditions  are  such  that  surface  evaporation 
from  the  tissues  exceeds  the  rate  of  moisture  diffusion  to  the  surface, 
the  surface  becomes  dry  and  hard  and  tends  to  retard  drying.  This 
condition,  known  as  case  hardening,  can  be  overcome  by  reducing  the 
temperature  of  the  air  or  by  increasing  the  humidity.  The  maximum 
rate  of  drying  is  attained  by  using  the  highest  temperature  which  will 
not  injure  the  prunes  and  sufficient  humidity  to  minimize  retarded 
drying  caused  by  case  hardening.  The  humidity  at  the  air-outlet  end 
of  the  drier  should  not  greatly  exceed  65  per  cent.  In  driers  employing 
recirculation  the  conditions  of  temperature  and  humidity  may  be 
largely  controlled  by  varying  the  recirculation. 

Some  operators,  observing  the  effect  of  increased  humidity  in 
reducing  case  hardening,  increased  the  humidity  of  the  air  in  the 
dehydrater  to  as  high  as  35-40  per  cent  at  a  temperature  of  165°  F. 
When  such  a  high  humidity  could  not  be  reached  by  recirculation 
alone,  they  increased  the  humidity  further  by  running  water  into 
the  heating  chamber.  While  such  high  humidity  very  largely  elimi- 
nated case  hardening  it  also  reduced  the  moisture  absorbing  capacity 
of  the  air,  especially  at  the  colder  end  of  long  dehydraters.  By  going 
to  such  an  extreme,  the  drying  time  of  the  prunes  was  increased,  the 
capacity  of  the  dehydrater  reduced  and  the  cost  of  operation  increased. 
Case  hardening  is  only*  a  temporary  condition  during  drying,  not 
necessarily  associated  with  injury  to  quality  and  since  it  disappears 
entirely  during  the  subsequent  binning  and  processing  of  prunes,  it  is 
not  a  condition  to  be  feared,  providing  the  flesh  around  the  pit  is 
sufficiently  dried. 

In  order  to  obtain  exact  information  on  the  relation  of  humidity 
to  the  drying  time  and  quality  of  prunes,  a  series  of  carefully  con- 
trolled comparative  experiments  was  conducted.  A  uniform  lot  of 
prunes  was  divided  into  three  parts  and  dried  separately  by  the 


Bull.  404] 


THE   DEHYDRATION    OF    PRUNES 


43 


counter  current  system  at  percentages  of  relative  humidity  of  10,  25 
and  40  per  cent  respectively,  referred  to  the  finishing  temperature  of 
165°  P.  All  other  variables  such  as  tray  load,  initial  and  finishing  tem- 
peratures, air  flow  and  drying  ratio  were  identical  for  all  three  lots. 

As  can  be  seen  by  reference  to  Fig.  13,  each  increase  of  15  per  cent 
in  the  finish  humidity  caused  an  increase  of  two  hours  in  the  drying 
time. 

The  prunes  finished  at  40  per  cent  humidity  showed  no  case 
hardening  and  considerable  stickiness,  those  at  25  per  cent  considerable 
case  hardening  and  slight  stickiness  and  those  at  10  per  cent  severe 
case  hardening  and  no  stickiness.  However,  all  case  hardening  dis- 
appeared after  the  prunes  had  equalized  in  bins  for  several  weeks. 


< 

2       3       4 

5 

7 

p 

9     T 

i     T 

T     T 

2       T3      T4      T5      T6      T 

7      T8      T9      2 

0     2 

t     : 

?.    ; 

S       PA       PR 

^ 

20 

\N 

V 

T 

V    OF 

H'JMftJlTY    OK 

30 

V 

+, 

RATE   0?   EVAPGKATIOS    OF  WATiS 

40 

1 

\ 

\ 

^ 

toi 

fro::  PH'JSts. 

PO 

h 

'   NX 

TO- 

fcfc 

fin 

^c* 

i^jk 

X* 

^ 

^ 

r^ 

70 

-<Oc 

fc 

V 

80 

! 

BMP 

SRaI'I 

fRE 

*<* 

V$o 

*- 

qo 

Start  -  135° 

Finish-   165* 

1         1 

TOO 

A.W.CHKUTi 


Fig.  12. — The  effect  of  humidity  on  the  rate  of  evaporation  of  water  from  prunes. 

In  order  to  ascertain  what  effect,  if  any,  the  humidity  had  on  the 
quality  of  the  prunes,  a  sample  of  each  lot  was  cooked  in  the  ordinary 
way  and  a  number  of  persons,  unaware  of  the  manner  in  which  the 
prunes  had  been  dried  were  asked  to  taste  the  samples  and  state  their 
preference.  The  consensus  of  opinion  was  that  the  prunes  dried  at  the 
lowest  humidity  had  the  best  flavor  and  those  dried  at  the  highest 
humidity  the  poorest  flavor,  although  the  lot  finished  at  25  per  cent 
humidity  was  judged  almost  equal  to  the  best.  Incidentally,  a  lot  of 
the  same  prunes  which  had  been  sun-dried  was  judged  poorer  than 
any  of  the  dehydrated  lots.  The  injurious  effect  of  the  high  humidity 
was  probably  due  to  the  much  higher  wet  bulb  temperature  to  which 
they  were  subjected,  as  explained  on  page  41. 

As  a  result  of  these  and  similar  observations,  it  is  recommended 
that  the  humidity  in  prune  dehydraters  be  not  allowed  to  exceed  25 
per  cent  at  a  temperature  of  165°  F.,  equivalent  to  a  wet  bulb  tempera- 
ture of  118°  F.     If  this  does  not  give  relief,  it  may  be  necessary,  in 


44  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

very  long  dehydra,ters,  to  reduce  the  number  of  cars  in  the  tunnel, 
thereby  increasing  the  temperature  and  de2reasing  the  humidity  of  the 
air  which  the  prunes  encounter  on  entering  the  dehydrater. 

Considerable  trouble  has  been  experienced  in  some  years  with 
bleeding  or  dripping  of  prunes,  especially  large  thin  skinned  varieties, 
at  the  cooler  end  of  long  dehydraters.  This  condition  is  usually  due 
to  too  high  a  degree  of  humidity  at  the  cold  end  of  the  drying  chamber, 
which  in  turn  is  caused  either  by  inadequate  air  flow  or  by  maintenance 
of  excessive  humidity  at  the  hot  end,  resulting  in  an  air  approaching 
saturation  at  the  other  end  of  the  dehydrater. 

The  remedy  in  the  first  case  is  to  increase  the  air  flow  by  increasing 
the  speed  of  the  fan.  An  even  lower  humidity  may  be  necessary  in 
some  cases  as  explained  below.  However,  exceptionally  low  humidity 
can  only  be  obtained  by  greatly  reducing  recirculation,  which  in  turn 
increases  fuel  consumption.  While  this  will  increase  the  power  con- 
sumption per  hour,  the  more  rapid  drying  will,  if  anything,  decrease 
the  power  cost  per  ton.  Moreover,  the  prevention  of  dripping  and 
sticking  will  reduce  the  labor  of  scraping  and  washing  trays. 

The  remedy  in  the  second  case  consists  in  lowering  the  humidity 
throughout  the  dehydrater  by  reducing  recirculation.  When  this  is 
done,  the  prunes  will  begin  to  dry  soon  after  entering  the  dehydrater 
and  dripping  will  be  minimized. 

DRYING   TIME 

The  time  required  to  properly  and  thoroughly  dehydrate  prunes 
has  been  the  subject  of  many  exaggerated  and  misleading  statements. 
The  required  time  may  vary  considerably  depending  on  the  condition 
of  the  fruit,  as  well  as  on  conditions  within  the  dehydrater.  The 
percentages  of  moisture  and  sugar  in  the  prunes  before  drying,  the 
size  of  the  prunes,  the  efficiency  of  dipping  and  the  amount  of  moisture 
remaining  in  the  prunes  when  removed  from  the  dehydrater  all  affect 
the  drying  time.  The  temperature,  humidity  and  volume  of  the  air 
passing  through  the  dehydrater  definitely  affect  the  rate  at  which 
moisture  is  evaporated. 

Drying  times  ranging  from  20  to  36  hours  have  been  observed  in 
all  leading  types  of  dehydraters.  In  no  case  has  a  drying  time  of 
less  than  20  hours  been  observed  except  where  partly  sun-dried  prunes 
were  finished  in  a  dehydrater.  Large  prunes,  counting  40  or  less  per 
pound  after  drying,  normally  require  28  hours  or  longer,  especially 
if  high  in  sugar  content.  Taking  as  a  representative  average,  plump 
French  prunes  with  a  drying  ratio  of  2y2  :  1  and  counting  50  to  60 


Bull.  404]  THE   DEHYDRATION    OF   PRUNES  45 

per  pound  when  dry,  the  average  drying  time  is  found  to  be  24  hours. 
No  dehydrater  manufacturer  can  conscientiously  claim  a  faster  drying 
time  nor  should  growers  use  a  lower  figure  in  estimating  the  size  of 
dehydrater  required.  It  is  neither  fair  nor  safe  to  base  the  capacity 
of  a  dehydrater  on  the  exceptionally  short  time  recorded  for  an 
occasional  car.  The  average  drying  time  for  all  cars  for  the  season 
is  the  only  reliable  index  of  capacity. 

STORAGE 

Moisture  Content. — The  proper  moisture  content  at  which  prunes 
may  be  safely  removed  from  the  dehydrater  and  stored  must  remain 
a  matter  of  judgment  and  experience  since  there  is  unfortunately  no 
simple,  quick  and  exact  method  for  determining  moisture  in  prunes. 
While  the  same  tests  commonly  used  on  sun-dried  prunes  (see  Bulletin 
388)  are  in  part  applicable  to  dehydrated  prunes,  certain  precautions 
must  be  observed.  When  prunes  are  removed  from  a  dehydrater,  they 
seem  moister  than  they  really  are  because  of  the  softening  effect  of 
the  heat  they  still  retain.  Consequently,  their  dryness  should  not  be 
judged  until  they  have  been  thoroughly  cooled.  Prunes  which  are  case 
hardened  seem  drier  than  they  really  are  and  should  be  cut  open  and 
the  flesh  around  the  pit  examined  for  its  dryness.  If  prunes  are  so 
moist  that  the  pit  can  be  freely  rolled  around  under  the  skin  between 
the  fingers,  they  are  not  sufficiently  dried.  The  pit  should  be  firmly 
held  in  the  flesh. 

Many  of  the  complaints  made  against  dehydrated  prunes  in  the 
past  can  be  traced  to  insufficient  drying  which  caused  packers  trouble 
in  storing  and  processing.  Prunes  containing  in  excess  of  26  per  cent 
moisture  will  eventually  mold.  However,  prunes  of  such  high  moisture 
content  will  not  withstand  binning,  grading  or  processing  without 
injury,  and  no  packing  house  should  receive  them.  In  order  that 
prunes  will  keep  well  for  a  long  period  in  bins  and  to  allow  for  the 
necessary  absorption  of  moisture  during  hot  water  processing,  prunes 
should  not  be  binned  with  a  moisture  content  in  excess  of  about  20 
per  cent.  However,  the  safe  moisture  content  varies  somewhat  with 
the  size,  variety  and  sugar  content  of  the  prunes  and  no  exact  moisture 
standard  can  or  should  be  given  at  this  time.  Operators  should  not 
be  influenced  by  the  exaggerated  claims  occasionally  made  by  over- 
optimistic  dehydrater  salesmen,  but  should  allow  the  prunes  to  remain 
in  the  dehydrater  until  adequately  dried,  regardless  of  the  time 
required. 


46  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Binning. — No  matter  how  carefully  a  dehydrater  be  operated,  the 
prunes  removed  therefrom  will  be  more  or  less  uneven  in  their  moisture 
content.  Consequently,  it  is  necessary  for  the  prunes  to  undergo 
equalization  of  moisture,  commonly  termed  sweating,  in  order  to 
insure  their  sufficient  and  even  drying  before  delivery  to  a  packing 
house.  The  importance  of  this  operation  is  best  illustrated  by  the 
following  quotation  from  Bulletin  No.  763  of  the  Dried  Fruit  Asso- 
ciation of  California: 

"It  has  been  brought  to  our  attention  that  dehydrated  prunes  in 
some  instances  are  being  delivered  direct  from  the  dehydrater  to  the 
packing  plant.  The  best  practice  requires  that  no  dehydrated  prunes 
should  be  received  by  any  packing  plant  until  after  such  prunes  have 
stood  in  the  bin  not  less  than  ten  days  to  two  weeks,  and  have  been 
allowed  to  pass  through  equalization,  and  you  are  requested  to  be 
governed  accordingly. ' ' 

Arrangements  for  removing  the  dried  fruit  from  the  trays  and 
transferring  it  to  storage  bins  vary  greatly  according  to  the  size  of 
the  plant.  In  small  plants  the  simplest  method  is  to  scrape  the  prunes 
into  rows  of  lug  boxes  on  the  floor,  which  when  filled  are  carried  to 
nearby  bins  and  emptied.  A  better  scheme  is  to  run  a  track  between 
the  rows  of  bins,  in  which  case  the  prunes  may  be  scraped  directly  from 
the  trays  into  the  bins.  The  prunes  should  be  allowed  to  become 
thoroughly  cooled  before  being  transferred  to  a  bin.  Poor  quality 
or  under-dried  prunes  should  be  removed  from  the  trays  before  the 
remaining  prunes  are  binned. 

In  larger  plants  much  labor  is  saved  by  moving  each  car  of  dried 
prunes  up  to  a  hopper  into  which  the  trays  are  scraped,  the  prunes 
being  distributed  to  the  various  bins  by  an  elevator  and  conveyor  as 
in  packing  houses.  This  system  is  ideal  for  custom  dehydraters  where 
many  lots  must  be  binned  separately. 

In  small  plants,  in  order  to  minimize  the  number  of  men  employed, 
it  is  generally  advisable  to  scrape  trays  for  half  the  day  and  dip 
prunes  the  other  half  day.  In  this  case  it  is  necessary  to  restack  the 
emptied  trays  on  trucks  standing  adjacent  to  the  point  where  needed 
for  reloading. 

Every  plant  should  have  sufficient  storage  space  for  the  entire 
season's  capacity  because  it  may  not  be  either  convenient  or  possible 
to  make  packing  house  deliveries  before  the  drying  season  is  over. 
It  is  well  to  turn  the  prunes  after  they  have  been  in  the  bin  for  a 
week  or  two.  If  a  layer  of  under-dried  or  moldy  prunes  is  revealed, 
they  should  be  carefully  removed  to  avoid  mixture  with  the  remaining 
prunes. 


BULL.  404]  THE   DEHYDRATION   OF   PRUNES  47 


SUMMARY    OF    OPERATING    METHODS 

In  conclusion,  the  following  summary  of  steps  in  the  dehydration 
of  prunes  may  be  considered  as  the  present  standard  based  on  years 
of  cumulative  experience : 

1.  Lye  dip  as  soon  after  harvesting  as  possible  and  rinse  in  fresh 
water. 

2.  Separate  into  two  or  more  size  grades  when  traying. 

3.  Enter  in  cooler  end  of  dehydrater,  preferably  of  air-blast  type, 
at  120°  F.  to  140°  F. 

4.  Finish  at  a  temperature  not  exceeding  165°  F.  and  a  humidity 
not  exceeding  25  per  cent. 

5.  Store  the  thoroughly  dried  prunes  in  bins  for  at  least  two  weeks 
before  delivery  to  packing  house,  turning  if  examination  reveals 
inadequate  equalization. 


SELECTED  EEFEEENCES 

1  Christie,  A.  W.,  and  Barnard,  L.  C. 

1925.    Principles  and  practice  of  sun-drying  fruits.     California  Agr.  Exp. 
Sta.  Bui.  388:   1-60. 

2  Christie,  A.  W.,  and  Eidley,  G.  B. 

1923.    Construction  of  farm  dehydraters  in  California.     Journ.  of  Amer. 
Soc.  of  Heating  and  Ventilating  Engineers.     29:   687-716. 

3  Hendrickson,  A.  H. 

1921.    Prune  growing  in  California.     California  Agr.  Exp.  Sta.  Bui.  328: 
1-38. 

4  Nichols,  P.  F.,  et  al. 

1925.    Commercial   dehydration    of   fruits   and   vegetables.      U.    S.    Dept. 
Agr.  Bur.  Chem.  Bui.  1335:    1-40. 
0  Eidley,  G.  B. 

1921.    Tunnel   dryers.     Journ.   Ind.   and  Eng.   Chem.     13:   453-460. 
6Wiegand,  E.  H. 

1923.  Eecirculation  dryers.     Oregon  Agr.  Exp.  Sta.  Circ.    40:    1-11. 

7  WlEGAND,  E.   H. 

1924.  Drying  prunes  in  Oregon.     Oregon  Agr.  Exp.  Sta.  Bui.    205:   1-26. 


